window: Don't stop irrelevant TOUCH_END events during window grab ops

During window grab ops we only react to touch events that have the
pointer emulating sequence, all other events should be propagated.

GNOME/mutter!427
3 jobs for !427 with fix-touch-window-dragging in 6 minutes and 6 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Review
passed #245197
check-commit-log

00:01:14

 
  Build
passed #245198
build-mutter

00:02:30

 
  Test
passed #245199
test-mutter

00:02:21