Add doc Porting to Nuvola Player 3

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Link use cases of Init. and Preferences Forms

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Link Nuvola Player Development forum in tutorial

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