broadway: use 'click-to-focus' approach instead of 'focus-follows-mouse'

broadway: use 'click-to-focus' approach instead of 'focus-follows-mouse'

The broadway backend would move the focus from one window to another based on
where the mouse was (i.e. 'focus-follows-mouse' approach). Handling the focus
this wait didn't play well with widgets which rely on focus-in-event and
focus-out-event, like the GtkEntry when using a completion popup window, see
e.g:
  https://bugzilla.gnome.org/show_bug.cgi?id=708984

So instead, setup broadway to require a click in a window to move the focus
(i.e. 'click-to-focus' approach):

 * The implicit GDK_FOCUS_CHANGE events that were generated upon reception of
   BROADWAY_EVENT_ENTER or BROADWAY_EVENT_LEAVE are removed.

 * The broadway daemon will now keep track of which is the focused window

 * Whenever the daemon detects an incoming BROADWAY_EVENT_BUTTON_PRESS, it will
   trigger the focused window switch, which sends a new BROADWAY_EVENT_FOCUS to
   the client, specifying which windows holds the focus.

 * Upon reception of a BROADWAY_EVENT_FOCUS, the client will generate a new
   GDK_FOCUS_CHANGE.

 * gdk_broadway_window_focus() was also implemented, which now requests the
   focus to the broadway server using a new BROADWAY_REQUEST_FOCUS_WINDOW.

This is based on an initial patch from Aleksander Morgado <aleksander@lanedo.com>.