This will be used right before handle_event() in order to filter
out events, useful to make the previous "no touchpad events" behavior
the default, and have gesture subclasses include manually the touchpad
events they handle.
There are two scenarios. A widget sub-class owns a GtkEventController
and passes itself to it, or a controller owned by something else is
passed a widget.
In the second case, if the widget is destroyed before the controller,
we will have a crash when destructing the controller because we will
be accessing invalid memory. Adding a weak reference on the widget
addresses that problem.
This leads to a crash in the first case. When the widget is getting
destroyed, it will drop the reference to its own controller. The
controller will skip touching the widget because the weak reference
would have turned it to NULL. However, when the widget sub-class chains
up to GtkWidget it will try to free all the controllers in its list.
Unfortunately, all these controllers have already been destroyed. So
we need to guard against this too.
https://bugzilla.gnome.org/show_bug.cgi?id=745225
Those might trigger the destruction of some widget that would dispose the
event controller while the event is still being handled, so keep an extra
ref on the controller during event processing.
This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.
Event controllers now auto-attach, and the GtkCapturePhase only determines
when are events dispatched, but all controllers are managed by the widget wrt
grabs.
All callers have been updated.
The propagation phase property/methods in GtkEventController are gone,
This is now set directly on the GtkWidget add/remove controller API,
which has been made private.
The only public bit now are the new functions gtk_gesture_attach() and
gtk_gesture_detach() that will use the private API underneath.
All callers have been updated.
