Derive all StaffGroup-like contexts from StaffGroup

Previously, there were many inconsistencies between the context types StaffGroup, ChoirStaff, GrandStaff and PianoStaff. They had differences in their lists of accepted contexts, and setting localAlterations or not. This lets ChoirStaff and GrandStaff inherit from StaffGroup (PianoStaff still inherits from GrandStaff) to keep all of them in sync.

StaffGroup gains a bunch of \accepts, so it accepts everything also accepted by Score.

This fixes a problem occurring when a TabStaff was included in a ChoirStaff. The latter did not accept it, causing a StaffGroup to be inserted in between, with its own bracket.

Closes: #6094 (closed)