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.
This prevents some of our generic object implementation tests
from working with gesture objects. Instead, add g_return_if_fail
checks in all the gesture constructors.
