Context mods stored in variable, can be inserted into \with or \context

Context mods stored in variable, can be inserted into \with or \context

-) Context-Modifications: create C++ class to store them
-) context modifications lists are stored in a dedicated simple scheme object
     (C++ class Context_mod)
-) Changes to the parser:
  -) \with blocks no longer return a simple list, but a Context_mod object
  -) context_modifications objects (stored in variables) are now also allowed
     with \with clauses
  -) context_modifications objects are also allowed inside \context

-) this allows us to rewrite \RemoveEmptyStaffContext (unfortunately with
   a little different syntax, since we no longer store \Staff inside the
	 \RESC command) so that it no longer erases previous settings to the
	 Staff context. Now, instead of
        \context { \RemoveEmptyStaffContext }
	 one can do
        \context { \Staff \RemoveEmptyStaves }
	 with the same effect and preserve previous changes to the Staff context.
	 (The same applies of course to \DrumStaff, \RhythmicStaff, etc. as well)

-) Adjusted engraver-init.ly and the regtests accordingly; Also added regtest
   that checks for RESC not discarding previous settings to the Staff context
-) Add convert-ly rule (\RemoveEmpty*StaffContext -> \*Staff \RemoveEmptyStaves)
-) Add scheme interface to extract context mods and add context mods with Scheme
-) Add changelog entry