Make the window labels visible at all times

When in overview, window labels flicker or are temporarily hidden on a
number of occasions - when simply clicking around the area the windows
are displayed in, dragging a window, sliding in the workspace list,
adding new workspaces etc. This patch makes the label for any window
visible at any given moment when in overview and the said window is
not being dragged around.

https://bugzilla.gnome.org/show_bug.cgi?id=644861