overview: Keep open when a Control key is held

It is useful at times to perform several actions that would usually
close the overview (for instance launching an application) at once.
Currently we allow this by dragging items to a workspace rather than
just clicking it, but it's an odd metaphor with its own set of
problems.
Introduce an alternative approach (inspired by file selection in
file managers) by keeping the overview open if a Control key is
held down.

https://bugzilla.gnome.org/show_bug.cgi?id=686984