renderer: Accept missing mime type in AVTransport.is_playlist()

"assertion 'mime != NULL' failed" is not useful here.

https://bugzilla.gnome.org/show_bug.cgi?id=711661
renderer: Add Next and Previous to CurrentTransportActions

https://bugzilla.gnome.org/show_bug.cgi?id=711661
Add milliseconds support in Rygel.TimeUtils time to string conversion

https://bugzilla.gnome.org/show_bug.cgi?id=711399
data: Remove allowedValueList from TransportPlaySpeed

AVTransport2 TransportPlaySpeed state variable is allowed to have
vendor specific values.

https://bugzilla.gnome.org/show_bug.cgi?id=710368
renderer: Add play_speed_to_double() to MediaPlayer

This can be useful for all player implementations.

https://bugzilla.gnome.org/show_bug.cgi?id=710368
renderer: Implement DLNA byte seeking

Adds X_DLNA_GetBytePositionInfo() method, the required state variables
and the missing SeekModes for Seek() to AVTransport2 implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=710368
https://bugzilla.gnome.org/show_bug.cgi?id=707058
https://bugzilla.gnome.org/show_bug.cgi?id=707059
https://bugzilla.gnome.org/show_bug.cgi?id=707541
renderer-gst: Enable more playspeeds

Negative playspeeds do not seem to work with common formats, so they
have not been enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=710368