When slipping meta strip, effect position was not updated. These updates
are limited to VSE core code, so function `SEQ_time_slip_strip()` was
added to resolve this issue.
This also simplifies operator code, so it does not have to deal with
recursion and doesn't need to hold quite detailed strip state for when
operator is cancelled.
Pull Request: https://projects.blender.org/blender/blender/pulls/113200
04efca91f9