userstudy: grab the pointer

Still losing the focus here sometimes (north of the window?) but at least on a
multiscreen setup I don't completely background anymore.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
A couple of rewordings after the first external study run

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>