Prohibit non-postevents returned by music functions to be used as post-events

Prohibit non-postevents from music functions to be used as post-events

For example,

{ c-\mark \default }

produced an invalid music expression more or less indistinguishable from

{ \mark \default c }

Worse is that things like

{ c-\tweak color #red { g } }

also got accepted.  While \tweak (and other music functions) may
indeed return either post-events or straight music expressions, there
should be checks in place that the usage corresponds to its type.


Unfortunately, already make test turned up points of conflict
addressed by the following additional patch:

Make \balloonText into post-event, \balloonGrobText into proper event

This is a stopgap measure for getting some consistency into music
function use.  The real fix would be to convert \ballonText and
\balloonGrobText to the same kind of interface that \footnote uses.
Indeed, it might be saner to retire the whole AnnotateOutputEvent
folderol and instead just use footnotes without footnote-text.


With this patch set, make test succeeds here.  I would not be
particularly surprised, however, if the documentation compile turned
up further problems.  After all, the purpose of the patch is to flag
previously unnoticed sloppy code, and our documentation contains a lot
of code.

[lemzwerg, 2013-03-16 15:57:07]

LGTM.

[janek, 2013-03-17 15:51:51]

commit message says:
> Prohibit non-postevents from music functions to be used
> as post-event

Do i understand correctly that this sentence is equivalent to "Prohibit
non-postevents returned by music functions to be used as post-events"?  If so,
i'd prefer that second wording; the original looks less straighforward to me.

thanks,
Janek