GtkGestureLongPress and GtkGestureMultiPress are both used
to replace it.

Instead do so on button release, or after text selection dragging
starts.

And delegate on GtkGesture::check(), it does deal better with
extra fleeting touches.

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.

This function sets the state on all sequences where it actually
can, just a helper function to avoid iterating over sequences
yourself.

This is so we don't have to specify the capture phase for
every controller. The default "bubble" value will be sane
on most situations.

This reverts commit 6511f411955adba4f960b8b07d39b93785afe33e.

GtkSettings::gtk-long-press-time is used to this effect.

This setting determines the duration of a button/touch press to
be considered a long press.

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).

Replace custom double click detection with multipress gesture

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.

This allows children to receive an emulated press, and handle it together with
the new touch that made the scrolledwindow sequence denied.

With gtk_gesture_get_point() returning events in
gtk_event_controller_get_widget() coordinates, we no longer need
to compensate for the overshoot here.

This helper function ensures the state is propagated for
all the active sequences in the given gesture.

Ensure that state being set on pointer emulating touches actually
gets propagated properly on widgets with gestures that only handle
pointer events.