gdk: Implement motion history as motion event data

gdk: Implement motion history as motion event data

In the motion compression phase the coalesced events will be saved
as a GdkTimeCoord on the motion event that shall be delivered.

For simplicity (and because history doesn't make much sense otherwise)
event history is only recorded while there are buttons pressed, this
also tidily ensures that those coalesced events would have the same
target widget on the gtk side than the delivered one, because of
implicit grabs.