These will be mutually exclusive with touch events, so it won't
be possible to trigger gestures through mixed input and whatnot.

The accounting of touchpad events is slightly different, there
will be a single internal PointData struct, stored in the hashtable
with the NULL event sequence/key (same than pointer events in
this regard), just that the events stored will be GdkEventTouchpad*,
so will hold information about all fingers at once.

But this difference is just internal, the GtkGesture API doesn't
make explicit assumptions about the number of points (the closest
to a per-point query API is gtk_gesture_get_sequences()). All
signals emitted just contain the last changed GdkEventSequence,
and API takes GdkEventSequences, so everything is consistent with
sequence=NULL for touchpad events.