Handle transfers of D-Bus name ownership

[Paul Kuliniewicz]

The NameOwnerChanged D-Bus signal is emitted in three possible circumstances:
 * A name is acquired
 * A name is released
 * A name is transfered from one owner to another

Panflute only handles the first two circumstances and ignores the third. This prevents Panflute from properly handling the case where two instances of the same D-Bus-using player are running: Panflute can get stuck paying attention to an instance that no longer exists.

Steps to reproduce:
1. Start VLC. Panflute starts paying attention to it.
2. Start VLC again. Panflute ignores it and pays attention to the first instance.
3. Quit the first instance of VLC.

Expected behavior:
Panflute switches to the remaining instance of VLC.

Actual behavior:
Panflute keeps trying to work with the first instance of VLC and ignores the remaining one. It continues doing so until the remaining instances of VLC are terminated.

Panflute needs to respond to NameOwnerChanged signals where ownership is transfered, and respond by paying attention to the new owner of the D-Bus name (here, org.mpris.vlc). This will ensure it always pays attention to the current owner of the name.

See also <a href="https://bugs.launchpad.net/panflute/+bug/596511/comments/1">this comment on a related bug</a>.