xwayland: Implement smooth scrolling

We don't even need to simulate button clicks; it's done automatically.
This also fixes scrolling in Qt5 apps.

Signed-off-by: Dima Ryazanov <dima@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit 81a51a6c)