overviewControls: Add the parent's box into the allocation

Currently, this is always 0, but this will change when we introduce
a custom layout manager into the story.
overviewControls: Mark the DashSlider as x_expand

Actors need to expand in order for them to their x_aligns to be taken
into account.
overviewControls: Remove dead code

The parent SlidingControl had an onOverviewShowing, but we had
overridden it with the same code in both subclasses. Just move it
back to SlidingControl.
overviewControls: Move slideOut on overview hide to the parent as well
overviewControls: Make visible and inDrag private

These don't need to be accessed from outside SlidingControl,
so don't let them be.
overviewControls: Make getSlide/updateSlide private
overviewControls: Always allocate the actor its full size

This is the new Clutter way -- if the actor wants to adjust its
alignment, it will do it itself with its alignment flags.
overviewControls: Move translation-x to SlideLayout

Not because ClutterActor is bad or wrong, but because I always get
confused on the difference, and having them both in SlideLayout
makes the code a bit easier to read and understand.