Blender Git Loki

Bugfix [#26438]: Hotkey conflict between add meta-strip and add marker

The sequencer made it's own copy of the Markers keymap, which was
inconsistent with the rest of Blender, making things confusing to use.
I've removed these duplicate keymap entries, and also changed the
conflicting hotkeys for Metastrips.

Metastrips now use the same hotkeys that their NLA cousins use:
Shift-G to add, Alt-G to remove; These were chosen since in user-
terms, metastrips are more like "strip groups"