Scroll button locking is an accessibility feature. When enabled, the scroll
button does not need to be held down, the first click holds it logically down,
to be released on the second click of that same button.

This is implemented as simple event filter, so we still get the same behavior
from the emulated logical button, i.e. a physical double click results in a
single logical click of that button provided no scrolling was triggered.

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