This is replaced by GtkGestureLongPress, and all its callers have
been updated to use the replacement.

This is somewhat analogous to scroll events on pointer devices, the
greater the velocity, the faster the spinbutton spins.

This gesture reports events on horizontal/vertical panning gestures.

This can be used to fetch the current velocity on update(), as opposed
to swipe() which happens after the sequence is finished.

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.