timeline: Avoid zooming in when setting the best zoom ratio
widgets: Fix the zoom slider not showing the initial zoom value
dogtail tests: Rename variable to clarify what it represents
dogtail tests: Reuse center method
medialibrary: Remove unused _trackElementAddedCb method
timeline: Raise an exception when the Timeline widget has no GES Timeline
timeline: Insert the clip at the end of the longest layer
timeline: Rename caps-locked local variable
timeline: Request at least the height of a pair of layers for the timeline
widgets: Specify parameter names when making pack_start calls
Remove unused receiver class
timeline: Remove unused SELECT_BETWEEN constant
Fix source code file headers
Remove unuseful instance module

This code never worked.
tests: Segfault by re-adding an asset to the project
ruler: Make sure we are using the same values we are reporting in the log message
ruler: Rename cryptic variable names
ruler: Use the correct constants to get colors
ruler: Simplify drawTimes a bit
ruler: Clarify what params setCairoColor handles

There are colors and colors, it seems.
Passing Gdk.Color objects is not handled yet.