SlideLayout is a fixed layout that takes care of requesting and
allocating the right sizes so its contents can slide horizontally as the
actor is resized.
Sliding is controlled with a slideX and slideDirection properties, which
do the right thing wrt. RTL automatically.
Also add a SlidingControl base class that will be used by the overview
to pack and slide the workspace thumbnail switcher and the dash.
https://bugzilla.gnome.org/show_bug.cgi?id=682050
