Blender Git Loki

Fix T79872: VSE - splitting strip shows the channel number when unused

This operator is dependent on mouse position (if the Use Cursor Position
option is used). The Channel property is irrelevant/unused in this case.
So it is not optimal displaying this property when calling this from the
menu (or even using the shortcut with default settings).

Now use a custom UI in the Adjust Last Operation panel in this case.
The properties are now drawn in relation to another then (Channel
underneath Use Cursor Position) next to some other minor layout
improvements.

Thx @HooglyBoogly for feedback (also providing UI code)

Maniphest Tasks: T79872

Differential Revision: https://developer.blender.org/D8625