Allow event functions from partial functions or partial markups

Allow event functions from partial functions or partial markups

This gives event functions such as ^\tweak font-size 2^\markup \huge \etc


Also contains commits:

Allow event functions from partial markup commands

This allows event functions such as ^\markup \huge #red \etc


Add Syntax::create_script

This generates post-events from generic Scheme expressions.

[thomasmorley651, 2018-03-16 19:19:15]

Can't review this one.
Though, from description, I'd like to suggest to put up an entry in `changes'.

[dak, 2018-03-16 20:40:39]

On 2018/03/16 19:19:15, thomasmorley651 wrote:
> Can't review this one.
> Though, from description, I'd like to suggest to put up an entry in `changes'.

Well, yeah.  Doesn't make a lot of sense without documentation and regtest
either.  I'll likely get them done separately.

[dak, 2018-03-20 12:56:37]

On 2018/03/16 19:19:15, thomasmorley651 wrote:
> Can't review this one.
> Though, from description, I'd like to suggest to put up an entry in `changes'.

Ugh.  Documentation for \etc is sort-of sparse, and the main problem for
"Changes" is "which Changes"?  \etc in general is a 2.20 feature and this fits
pretty well within that framework.  But it's as to yet not tested all that much.

[thomasmorley651, 2018-03-20 20:31:48]

On 2018/03/20 12:56:37, dak wrote:
> On 2018/03/16 19:19:15, thomasmorley651 wrote:
> > Can't review this one.
> > Though, from description, I'd like to suggest to put up an entry in
`changes'.
> 
> Ugh.  Documentation for \etc is sort-of sparse, and the main problem for
> "Changes" is "which Changes"?  \etc in general is a 2.20 feature and this fits
> pretty well within that framework.  But it's as to yet not tested all that
much.

Apart from wrong estimations (including my own), I don't remember any reported
problem with \etc.
So I vote for putting all in 2.20.

[Carl, 2018-03-20 20:35:03]

On 2018/03/20 20:31:48, thomasmorley651 wrote:

> problem with \etc.
> So I vote for putting all in 2.20.

I concur.