Rename Nuvola.PlayerPrototype to MediaPlayer

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Nuvola.MediaPlayer: prefix private props with _

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Google Play: fix extra actions

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>