Operator logic is limited to iterating over selection and executing
same code as python API does.
- No attempt to preserve effects is made, but this can be re-introduced. Dependant effects are deleted.
- No attempt to change meta strip boundaries, but this can be re-introduced as well.
- Python API calls delete function with current seqbase rather than topmost sequence. This will allow to delete strips in meta strips, though meta has to be viewed first, so it's still quite cumbersome. I could look for strip in metas if it could not be found in topmost seqbase.
Partially fixes T73828