Fix popovers to properly gain focus when clicked
inside an unfocused window.

We use the GTK_PHASE_CAPTURE of the 'pressed' event
to early detect that the popover is being clicked
inside an inactive window, this allow us to present
the window (and be focused) before the normal signal
handlers for the popover click/pressed events are run
which would ultimately give focus to popover widget.

This fix works for both modal and 'non modal' popovers
when being clicked inside unfocused windows.

Fixes issue #1871