timeline: Fix ctrl+click clears whole selection

Thejas Kiran P S requested to merge thejaskiranps/pitivi:selection-fix-2 into master

set_selection() was called after we created a temporary GES.Group which is used to hold selection elements. This temporary group was interfering with our selection logic and caused this bug. Now the group is created only when the mouse starts moving.

Fixes #2433 (closed)

Merge request reports