Move the tracking of the number of touch sequences on the screen to
determine whether the sequence is pointer emulating from
MetaGestureTracker to ClutterInputDevice. To do this, add new API to get
the number of touches to ClutterInputDevice and remove the API from
MetaGestureTracker.

This is a first step for removing the sequence tracking inside
meta-gesture-tracker, also it makes the check more clear since we can
compare the number against 1 now (the number of sequences of the input
device is already updated at this point).

GNOME/mutter!790