overviewControls: Correct the use of x2 in SlidingControl

The x2 here needs to be more than just the width; it needs to
be added onto the x1.

https://bugzilla.gnome.org/show_bug.cgi?id=694881
overviewControls: Clarify some code with a comment

translationX is sort of a bad name, since it confuses with the
actor's translation, which is used for sliding without allocation.

https://bugzilla.gnome.org/show_bug.cgi?id=694881
overviewControls: Don't try to align something sliding to the right

0 is already the correct value.

https://bugzilla.gnome.org/show_bug.cgi?id=694881
overviewControls: Don't use the child's preferred size to slide from

In order for the workspace thumbnails box to have the correct size,
we need to constrain the width of the thumbnails box to the height we're
given, instead of assuming an unlimited height.

https://bugzilla.gnome.org/show_bug.cgi?id=694881
workspaceThumbnail: Drop the _background hack

The _background hack was added because the old way the zooming animation
worked, it set the allocation of the workspaces view and thumbnails box
to the final position and used animations to smoothly animate.

During the 3.6 cycle when we added the new search view, Cosimo changed the
way the zoom animation works so that rather than set the final allocation
and animate, we actually do adjust the allocation of the workspaces view
and thumbnails box.

So, as the hack is no longer necessary, we can drop it.

https://bugzilla.gnome.org/show_bug.cgi?id=694881