Blender Git Loki

Bugfix [#25597] Grease Pencil crash when undoing during a Sketching
Session

As the key combination for undo was unhandled by Grease Pencil
operator and allowed to execute, some of the lingering Grease Pencil
data would get corrupted by undo as some flags may still have been
set.

This commit attempts to fix.workaround this problem by catching undo
events, using the internal "delete last stroke" functionality to
emulate undo-like behaviour as expected but without the associated
risks. The underlying functionality used was already part of the
original 2.4 implementation, but was exposed via the GUI instead there
where it was less useful.

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Other tweaks related to Grease Pencil:
1) Spacebar can be used to end Sketching Sessions too now
2) Grease Pencil animation editor now displays GP datablocks in light
blue (i.e. "sub-id") colours as per dopesheet instead of them being
presented like groups. This better reflects their true nature.