Logic was broken into finding gaps and ofsetting strips. Functions were modified so they work on explicitly defined seqbase, so they can be used as python API functions. Functional changes: - Improve performance by calculating gap length and offseting strips at once. Previously strips were offset by one frame. - Calculate gap from start frame. Previously gap was considered only inbetween strips. Reviewed By: sergey Differential Revision: https://developer.blender.org/D9730