Provide music functions property-{override,revert,set,unset} (issue 251130043 by address@hidden)

[dak]

Reviewers: ,

Message:
Well, the consideration for choosing a hyphenated name here instead of
camel case (namely property-override instead of propertyOverride) was
that the main usage was from Scheme anyway.  But since it is a proper
music function nevertheless, I now tend to think that I should not
diverge from the normal naming scheme and change that to
propertyOverride etc.  Yes, it will look a bit out of place in Scheme
code but so do all the other music functions consisting of more than one
word.  So I'll be uploading an appropriately changed version next.

Description:
Provide music functions property-{override,revert,set,unset}

The previous music functions `override' and `revert' (intentionally
colliding with the respective keywords) are removed.  It would have been
too confusing to have collisions for `set' and `unset' as well which are
just too similar to existing Scheme constructs.

A convert-ly rule for the previous calls override/revert is not
attempted: they have not yet been documented and have been available
only for one unstable version.  Whoever happened to use them already is
capable of replacing them.

Provide check-context-path function

Please review this at https://codereview.appspot.com/251130043/

Affected files (+103, -55 lines):
  M ly/music-functions-init.ly
  M scm/music-functions.scm