Add 'update_properties' signal and obs_source_update_properties call

obs_source_update_properties should be called by sources when property
values change, e.g. a capture device source would use this when it
detects a new capture device (in case its properties contain a list of
available capture devices or similar)
Handle 'update_properties' signal in window-basic-properties
Implement 'update_properties' for syphon
Improve inject availability detection