overview: Try to do the right thing related to XDnD

Rather than expose a dizzying array of methods related to managing
state that require infecting every user of the overview methods, try
to do the sensible and smart thing internally. Now, the overview
itself tracks when XDND drags start, and simply calling show, hide or
toggle while an XDnD drag is in effect will show the overview, and
will only take the grab until after the XDND drag ends.

https://bugzilla.gnome.org/show_bug.cgi?id=663661