overview: Fix stuck grabs when mashing the overlay-key

overview: Fix stuck grabs when mashing the overlay-key

When pressing the overlay key three times, things went like this:

  * show(), push a modal
  * hide(), will pop a modal after hiding is done
  * show(), push a modal

Thus, when the showing is done, and then it activated the hiding,
it popped one modal, but not the other. This patch changes things
to be:

  * show(), push a modal
  * hide(), will pop a modal after hiding is done
  * hide(), no-op

That is, mashing the overlay-key when it's showing will always make
it hide, not mashing an odd number of times.

https://bugzilla.gnome.org/show_bug.cgi?id=688589