Draft: Controllabe Gap

We are working on issue 2548, which is clip gap manipulation.

Currently, Pitivi does not have a method to manipulate gaps
between the clips. Issue 2548 is a requested feature change,
which is the reason for development.

The feature we have implemented is incomplete, but currently
allows for selecting and deleting gaps between clips. You can
only select and delete one "FakeClip" at a time. "Undo"
functionality is partially implemented, but currently
incomplete. We have also contributed documentation of the
gap deletion feature to the "Using Clips" section of the
help files in the User Manual.

GNOME/pitivi#2548