Ctrl+Click clears the selection instead of unselecting a single clip

Steps to reproduce:

• Create a new project
• Import a video file,
• Add two clips to the timeline,
• Select both using ctrl-click
• Ctrl-click one of them, notice both are deselected when only the clicked clip should have been.

A unittest should be added. The toggle_clip_selection which simulates the Ctrl+click on a clip should allow writing a test easily.