Introduce StaffCollection context

Previously, all staff-grouping contexts had layout implications: They changed vertical spacing, introduced system start delimiters, etc. Yet sometimes it is desirable to group staves only logically, for example in order to apply shared overrides/settings or to let a specific engraver act on them.

The new StaffCollection context provides such a purely logical grouping of staves. The StaffGroup (previously used as a "master" context for grouped staves) is now derived from StaffCollection by adding span bars (and their stubs), a system start delimiter, vertical alignment defaults etc.

This MR does not contain documentation yet, as it seems reasonable to settle on the implementation first. For background and discussion, see #6493.