timeline: Get rid of some clip animations

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719348
mainwindow: Remove a vbox element which is not useful
mainwindow: Consistent imports and move dead code around
clipproperties: Fix effect configuration UI destruction

The second resetKeyframeToggleButtons call was raising an exception.
medialibrary: Fix asset removal

    if paths is None or paths < 1:
TypeError: unorderable types: list() < int()
previewers: Fix initial value of AudioPreviewer.lastUpdate
video previewer: remove unused self._callback_id
previewers: Always set lastUpdate when computing geometry
previewers: Avoid removing sources already running

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726904
mainwindow: Better default placement for panes

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=702507
dogtail: Fix test_backup
timeline: Add spacing between the icon and the label of the zoom button
bin: Allow passing arguments to autogen per module
bin: Fix gobject-introspection building
clipproperties: Rename method
clipproperties: Fix effects tooltips
clipproperties: Disable sorting of the clip's effects