timeline.py: Add a clip looping feature

Currently the only way to loop a clip is by copy and pasting.

There is no way to easily loop a clip multiple times in Pitivi.

Unable to finish fraction amount looping.

We have added a loop button which loops the copy and pasting process.

https://tree.taiga.io/project/sgittelman2-soft-261-module-2-team-10/epic/1