st-widget: Sort actors by distance from the focused actor

Sorting actors by the distance in the axis of movement first and against
the axis otherwise means that if we have a situation like:

  A      F
   B

where "F" is the focused actor, and it slightly overlaps with B vertically,
then we'll choose "B" to go left, rather than "A", which is most likely
what the user intended.

This is especially apparent in the overview where slight window size
differences mean we might not get an exact grid shape.

https://bugzilla.gnome.org/show_bug.cgi?id=644306
workspace: Implement key navigation on the workspaces page

Simply use St's existing key navigation system by making all the window
clones StWidgets, and making the WorkspacesView a focus group.

Since the workspace view is effectively "fake", we need to add a focus
delegator so that when key focus is assigned to the fake workspaces page,
we can keynav inside it properly.

https://bugzilla.gnome.org/show_bug.cgi?id=644306
workspace: Grab the key focus when hovering over a window

This is simply an experiment. I'm not sure I like the result, but it was
worth trying out regardless.

https://bugzilla.gnome.org/show_bug.cgi?id=644306
viewSelector: Give the active page key focus when it is shown

Rather than implement special focus policies like only allowing keynav
when pressing down, simply give the active page key focus when entering
the overview.

This may break stuff, as it's somewhat of a tricky patch to get right.
Testing this one would be super appreciated.

https://bugzilla.gnome.org/show_bug.cgi?id=644306