WIP: validate: Override edit-container

In that override the edition is done faking UI events/signals to do the
timeline edition.
Port the timeline to Gtk+ and remove clutter dependency!
FIXUP gtktimeline: Fix CONTROL_WIDTH + enhance timeline and layer expanding behaviour
Add FIXME
GTKTIMELINE: DND fixes++
GTKTIMELINE: Minor renamings
GTKTIMELINE: Fix snapbar
WIP
WIP
WIP
WIP
WIP