D-Bus improvements

* Implemented SetPosition in D-Bus, this allows external programs to jump to a specific position
* Fixed Seek so that it obeys the passed relative seek value, as per MPRIS
* Added Position and Duration properties which return the current position and media duration in microseconds
* fixed some overflow errors by using int64_t instead of long, position was wrapping at around 31 minutes
* A little extra debug logging for D-Bus method calls, useful when trying to determine why it's not responding to a method
* Added a sample D-Bus control shell script
* Explicitly specify KeyConfig enum constants, in case external programs depend on them
* Changed D-Bus session handing so a single dbus-daemon is started the first time omxplayer starts, and continues running forever
* Added argument --dbus_name to override the default org.mpris.MediaPlayer2.omxplayer