Listen for notify::sequence-state-changed on the controller, so the
only way to manipulate a sequence state are gtk_gesture_set_sequence_state()
and gtk_gesture_set_state().
Also, make use of gesture groups, so the sequence state is set at once
on all gestures pertaining to a single group. Within a widget, if a sequence
is claimed on one group, it is made to be denied on every other group.
This API eliminates the need for overriding
GtkWidget::sequence-state-changed virtually everywhere. Grouped
gestures share common states for a same GdkEventSequence, so the
state of sequences stay in sync across those.
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.
