The slideX property controls the allocation of the view selector; since
we now know when there are no visible views from the page-empty signal,
we can use it to set the full slideX for the next page at that time,
allowing the new view to fade in with the right width.
This allows us to use simple x translations for the side components when
switching pages, keeping the noise due to resizes at the minimum.
The slideX resize for now is kept for DnD, and will always be needed for
the thumbnails box when showing the windows page.
https://bugzilla.gnome.org/show_bug.cgi?id=693924
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
