Get session bus address from X11 and follow autolaunch spec

[acrisci]

According to the spec, we should be getting the session bus address from the window properties of an X11 selection owner and fall back to the filesystem if that is not successful.

If the still cannot connect to the server, the spec says we should launch our own dbus server and take the X11 selection.

However, success in opening this file MUST NOT lead to the conclusion that the server is running. Thus, a failure to connect to the bus address obtained by the alternative method MUST NOT be considered a fatal error. If the connection cannot be established, the implementation MUST proceed to check the X selection settings or to start the server on its own.

If the implementation concludes that the D-Bus server is not running it MUST attempt to start a new server and it MUST also ensure that the daemon started as an effect of the "autolaunch" mechanism provides the lookup mechanisms described above, so subsequent calls can locate the newly started server.