Add partial \override and \set commands

Add partial \override and \set commands

This allows \set and \override commands to be stopped short after
their `=' sign in order to be used in the manner of partial music
functions.

In order to make this currently reviewable, this is based off the
combined patches for issue 4488, issue 4486, issue 4487, issue 4498
(all currently on countdown) which form the first patch in the
Rietveld review (it's more than a simple rebase since issue 4488 in
particular allows simplifying the parallelly reviewed issues).


Also contains marginally related commit:

Reorganize argument list for Syntax::partial-music-function


This one should have likely been done from the start for issue 4487
but I don't want to stop the countdown for this cleanup.

[dak, 2015-07-14 15:58:33]

Actual patch for review