popupMenu: Overwrite ongoing animations when calling close repeatedly

Currently close() is a no-op when the menu has already been closed.
However, repeated calls could pass different animation parameters.
For instance in the user menu, we try to hide the menu immediately
before locking the screen, to avoid the popup jumping across the
screen while fading out - as we do this from the corresponding
item's activate handler, the closing is still animated if the menu's
own handler (which requests a full animation) is run first.
Fix this by changing close() to overwrite ongoing animations before
bailing out early.

https://bugzilla.gnome.org/show_bug.cgi?id=686484