The former can be called individually on each sequence, and the latter will
always call the former on all currently active sequences, so only implementing
resetting on cancel() works for both cases. Also, chain up on subclasses
implementing cancel.
This fixes clicking on nautilus' file list after popping up a menu, as broken
grabs are one of those situations where sequences get cancelled individually,
the "current button" wasn't properly reset, and further clicks with button != 3
were ignored.
All "exclusive" gestures listen for either pointer events, or
"pointer emulating" touch events, so only a single sequence at
a time can make these run.
That may happen separately from grab-notify, and also due to external
reasons, so ensure all sequences are cancelled if a grab is taken
in some GdkWindows that would obscure events on the controller.
