Make \time work with \tweak and \footnote

Make \time work with \tweak and \footnote

The way this is done is to let a TimeSignatureMusic expression expand to
produce a TimeSignatureEvent that can be listened to.  The
Time_signature_engraver _does_ listen to this event and uses it to set
the origin for TimeSignature grobs created in the same time step.
Another effect is that any warnings associated with a TimeSignature grob
will be able to point to a source location in case that the respective
change of timing parameters can be traced to TimeSignatureMusic in a
context below the context of the Time_signature_engraver creating the
TimeSignature.

Since events that can be listened to create a Bottom context implicitly,
this causes a difference for cases like

\score { { \time 3/4 \skip 2. r2. } }

Previously, Staff (and Voice) contexts were only created _after_ the
\skip completed (the timing parameters are set in the Timing context,
equal to Score by default), making the time signature appear only in
measure 2.  Now the implicit creation of a Staff (and thus a
Time_signature_engraver) in the course of creating a Bottom context (in
this case a Voice) makes the time signature appear right away.

If one uses a separate non-printing context for containing timing info,
one has to make sure that any implicitly created Bottom context is
non-printing as well.

[Dan Eble, 2014-09-28 18:14:12]

I won't comment on the code, but I will confirm that this has the desired effect
of reporting the input location for the warning added in
<https://codereview.appspot.com/143450043/>.  Thank you.

[Dan Eble, 2014-09-30 03:50:30]

\compoundMeter should create a TimeSignatureEvent too.  I managed to do it by
incorporating \time into the definition of \compoundMeter, but perhaps you have
a better idea.

[dak, 2014-09-30 04:54:22]

On 2014/09/30 03:50:30, Dan Eble wrote:
> \compoundMeter should create a TimeSignatureEvent too.

[Slaps forehead]

> I managed to do it by
> incorporating \time into the definition of \compoundMeter, but perhaps you
have
> a better idea.

I'll take a look.

[dak, 2014-09-30 09:07:26]

On 2014/09/30 04:54:22, dak wrote:
> On 2014/09/30 03:50:30, Dan Eble wrote:
> > \compoundMeter should create a TimeSignatureEvent too.
> 
> [Slaps forehead]
> 
> > I managed to do it by
> > incorporating \time into the definition of \compoundMeter, but perhaps you
> have
> > a better idea.
> 
> I'll take a look.

Huh, what a crock.  I think the most reasonable course is to route
\compoundMeter through TimeSignatureMusic as well.  Tweaking a stencil on a
\time music would not work in those cases where tweaks don't work right now,
like

\score {
  << \new Staff { \tweak color #red \time 3/4 c4 4 4 }
     \new Staff { e4 4 4 }
  >>
}

where the color red will only appear on the first time signature (the second one
is generated from the same timing variables but does not get to see the
TimeSignatureEvent which is only announced in the first Staff, so the cause of
its TimeSignature grob remains unset).

So one still needs to record the stencil change in Timing itself rather than
route it through a tweak of TimeSignatureEvent.

[Dan Eble, 2014-09-30 23:04:00]

On 2014/09/30 09:07:26, dak wrote:
> On 2014/09/30 04:54:22, dak wrote:
> > On 2014/09/30 03:50:30, Dan Eble wrote:
> > > \compoundMeter should create a TimeSignatureEvent too.
> > 
> > [Slaps forehead]

Take a deep breath. There are also \overrideTimeSignatureSettings and
\revertTimeSignatureSettings, and examples in the NR that set
timeSignatureFraction directly.

I'm grateful for your attempt to help, but I wonder whether now is the right
time to touch this hodgepodge.  For my mid-measure time-signature warning, I
could content myself with printing the current bar number and the before/after
fractions.

[dak, 2014-10-01 05:26:42]

On 2014/09/30 23:04:00, Dan Eble wrote:
> On 2014/09/30 09:07:26, dak wrote:
> > On 2014/09/30 04:54:22, dak wrote:
> > > On 2014/09/30 03:50:30, Dan Eble wrote:
> > > > \compoundMeter should create a TimeSignatureEvent too.
> > > 
> > > [Slaps forehead]
> 
> Take a deep breath. There are also \overrideTimeSignatureSettings and
> \revertTimeSignatureSettings, and examples in the NR that set
> timeSignatureFraction directly.
> 
> I'm grateful for your attempt to help, but I wonder whether now is the right
> time to touch this hodgepodge.  For my mid-measure time-signature warning, I
> could content myself with printing the current bar number and the before/after
> fractions.

Setting timeSignatureFraction directly is not expected to be tweakable, and
\overrideTimeSignatureSettings and its revert does not even have an immediately
visible effect.  They are not a concern.

I am not predominantly helping you here: that "why isn't there a warning
location" was just an incentive.  Regarding the "is this cost reasonable" angle,
I am more worried about the warning as such rather than the TimeSignatureEvent. 
But then the warning is your project and you are free to pull the plug on it any
time you want.  Being able to tweak \time is independently useful, and it is
half the job of getting another @knownissue out of the manual (the other half
being clefs if I remember correctly).