A multipress gesture is used to control all this, replacing
single/double click custom code, and triggering window dragging
when the multipress is stopped, yet active (ie. the sequence remains
pressed).
This gesture is used by gtk_drag_source_set() to determine
whether dragging moved past the threshold. The gesture events
are handled via the usual ::event callbacks, so we don't mess
up with callers expecting that to happen in a signal handler.
If the sequence gets claimed somewhere else in the event widget
stack, the DnD gesture will be cancelled.
With gtk_gesture_get_point() returning events in
gtk_event_controller_get_widget() coordinates, we no longer need
to compensate for the overshoot here.
GtkEventController may be certainly useful to keep event
handling self-contained in other places than gestures, but
the current widget API is highly related to gestures, so
just using GtkGesture as the argument there will be quite
more convenient. The other places where GtkEventController
make sense as a base object will better provide their own
hooks.
Gestures attached with this phase will expect callers to have it
receive events through gtk_event_controller_handle_event(), but
the gesture will still be notified of sequence state changes,
grabs, etc...
Translate events meant for other widgets/windows, so gtk_gesture_get_point()
always returns coordinates based on the gtk_event_controller_get_widget()
allocation.
If a gesture has denied sequences (so those are presumably handled above/below
the widget), it shouldn't attempt to handle extra touches, even if those end
up matching the expected number of touches.
Gestures should always receive one of such events in order to be activated,
and the propagation mechanism will ensure they do so if the original event
was caught up the widget hierarchy by another gesture that is now declining
the sequence.
If the captured touch begin or button press event have been consumed
for the given sequence, propagate it upwards if the sequence goes from
claimed to denied, so the widgets on the way to the event widget receive
a coherent event stream now that they're going to receive events.
