gdk: Emulate motion events when requested

gdk: Emulate motion events when requested

GtkRange was using GDK_POINTER_MOTION_MASK, and it was not
getting any emulated motion events, because we only translate
from GDK_BUTTON_MOTION_MASK to GDK_POINTER_MOTION_MASK, but not
the other way around, and emulated_mask only had
GDK_BUTTON_MOTION_MASK in it. Now we put GDK_POINTER_MOTION_MASK
in emulated_mask and successfully match for windows that
have GDK_POINTER_MOTION_MASK or any of the button motion masks
selected.

This fixes range sliders not following the finger and jumping
to the last position upon release.