evdev: hook up left-handed configuration option

evdev: hook up left-handed configuration option

Two separate flags needed, want_left_handed and left_handed to avoid switching
to left_handed while a button is still down. Since each backend has a
different way of determining whether buttons are down, let them set a function
to do exactly that. Then call that function whenever a button release event is
posted to switch the device to right/left-handed if applicable.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>