Allow getting player state in any state

We can call xmp_get_player() to retrieve the player state even if
the current state is not "playing", otherwise it won't be a very useful
call.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update API documentation

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add common calls for extras

Keep extras calls separate, later we can generalize it using registered
extra drivers.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>