pxgsettings: use the correct syntax to connect to the changed signal
pxgsettings: use the correct syntax to connect to the changed signal

As it is a detailed signal, it only makes sense to append a :: when there is a specific property to target.
It used to be accepted but triggers a runtime warning with latest GLib.

libproxy/libproxy,horar/libproxy,horar/libproxy,libproxy/libproxy,libproxy/libproxy,horar/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,horar/libproxy,horar/libproxy,horar/libproxy,horar/libproxy,libproxy/libproxy