timeline: Connect button press/release to the timeline, not the whole stage

Signal handling is then more specific, preventing useless (and wrong) stuff
from happening when clicking layer controls instead of the timeline canvas.
This is in preparation for making focus-based timeline actions sensitivity work.
pitivi: Rely on timeline widgets' focus to set actions/shortcuts sensitivity

This greatly simplifies the code (present and future) and makes the whole app
incredibly more reliable when it comes to preventing "dangerous" timeline
shortcuts (such as Delete/Spacebar) from interfering with other GTK+ widgets.

Fixes bug #707828