Adds a simple signal reference counting function
(signal_handler_connect_ref) that makes it so that signals keep the
handler around until the all the signal itself is disconnected. This
prevents potential crashes where a signal might try to disconnect after
a handler has already been destroyed (typically in C++ with
OBSSignalHandler helper objects, where destruction isn't guaranteed to
be predictable).
This also modifies OBSSignalHandler to use the reference-counting
connections.
