combobox: popdown the combobox when on 'grab-broken-event'

combobox: popdown the combobox when on 'grab-broken-event'

And also explicitly remove pointer/keyboard grabs from the display.

Whenever the grab is reported lost, we should popdown the combobox, so that the
GDK_WINDOW_TEMP window is hidden and removed from the toplevel, as done with
the menu for example.

Leaving the GDK_WINDOW_TEMP window open when re-activating the application
triggers several issues in the win32 backend, due to restacking windows of the
non-toplevel group into the toplevel group:

https://bugzilla.gnome.org/show_bug.cgi?id=695200