add sensor_act and standardize sensor toggling with it (unless used in compat mode)
position G_FireTarget's default case outside the switcg, so we can still pass partly handled cases to the act function
abstract partly toggling and on/off into global firing mechanism
add explicit USE action
this will in compat mode act simalar as ACT on many entities,
but will change in the future for new entities
and already has a different effect on some buildables