Issue 5362: Generalize context searches (Take 2)

https://sourceforge.net/p/testlilyissues/issues/5362/

1: Add Context::matches ()

This method answers the central question of a context search: is this
what I'm looking for?  Its use will increase in later commits.

2: Generalize context searches

Combine code for \new, \context, and various internal searches.  This
includes a small block of disabled code which will fix
input/regression/context-find-parent.ly once it is enabled.

input/regression/context-descend-only-not-found.ly shows that a defect
in the descend-to-context function has been fixed: although the
previous implementation never changed context outside the current
subtree, it allowed creating a new context outside the current
subtree.

3: Change descend-only to search-direction

It is now a direction rather than a boolean.

There was a previous review for this issue that had more stuff in it:
https://codereview.appspot.com/344050043/

[Dan Eble, 2020-01-29 20:40:11]

Generalize context searches

[Dan Eble, 2020-01-29 20:40:44]

Change descend-only to search-direction