Some usb-modes / events do not have qt/qml compatible symbolic names and it
is not possible to tell apart mode change notifications from usb events that
do not affect current mode.
Add qt/qml constants for all built-in usb modes / events known to usb-moded.
Add methods that can be used for telling apart events from mode changes etc.
Signed-off-by: Simo Piiroinen <firstname.lastname@example.org>