Improves slightly the [Registered player] table

It looks like we need a deeper thinking about the form's usability and performances at the same time. Let's first consider that an added player does the job just fine for now.
commenting the xml format
Starting the [known players] table

Done:
- it's here.
- adding a player will add a row to this table
To do:
- players.xml management (load/save)
- checks (if player is already in the file, checks on player's inputs)
- Player declaration should be filled when pairing algorithm will be there.