When strip is added, operator first finds "best" channel to place a
strip in, then adds new strip to that channel and finally does check if
the strip is overlapping other strips. If it does, it handles the
overlap.
To prevent adding strip to locked or muted channel, fix has to be done
at 2 places:
- `sequencer_generic_invoke_xy_guess_channel` which finds the "best"
channel
- `transform_seqbase_shuffle_ex` which handles overlap.
Further check for free channels was added to all strip add operators.
If there is no space above channel, that user selected, error is
displayed and operator is cancelled.
Note, that `transform_seqbase_shuffle_ex` is used only for resolving
overlaps where strip position in time must stay constant, so it moves
the strips in Y axis. This PR does not affect user selectable
`overlap_mode`.
Pull Request: https://projects.blender.org/blender/blender/pulls/136016
c4eab49b9c