Implement XML constructor for Player

[rexim]

Split #12

It should construct the player from the XML node according to the XML schema level.xsd. It should also check if the corresponding node exists.

https://github.com/rexim/beatwave/blob/dc8ab4f0bfcf2c592b87f1bbe9df6b12f3297d25/src/beatwave/player.cpp#L11