applets/taskmanager: Make MPRIS-related stuff more reliable
When a source is added, always connect it. This prevents an issue where closing and reopening a media-player doesn't have controls the second time around.
Steps to reproduce the bug:
- Open a media player, like Elisa
- Note that it has controls
- Close it and open it again
- Controls aren't there anymore
Or with browsers: close a video tab and open another.
This issue didn't show up in the Media Player applet because it always connects to the multiplexer source, which provides a proxy for the missing source.