add subsurface-helper.cpp to share functions between mobile and desktop
move mobile qml registrations to a shared function (avoiding differences in registrations)
Target is to replace current subsurface-desktop-main + subsurface-desktop-helper and
subsurface-mobile-main + subsurface-mobile-helper with
subsurface-*-main + subsurface-helper
Signed-off-by: Jan Iversen <jani@apache.org>
