WIP: Shared port

This makes a spice-streaming-agent instance to close its streaming port when the session it's running in is not active. when the session becomes active again it opens the port and start to run. In order to be aware to session activeness it uses the system dbus interface. This works only when dbus and all the required interfaces are available. This approach does not use management daemon and each user run its own instance of spice-streaming-agent. This is fairly simple implementation but seems to work well although it still require more work, please share your thoughts about the basic concept, design or any other suggestions or comments. (I disabled the cursor thread for now, will be fixed in separate)