Remember player identity

[supertuxkart-sourceforge-migration]

Author: auria

Atm we ask for player name when opening the game. This gets annoying. A few ideas :

1) add a checkbox "remember be" 2) on the main menu, show "Playing as Auria", and a button "Switch player"

Migrated-From: https://sourceforge.net/apps/trac/supertuxkart/ticket/525

[supertuxkart-sourceforge-migration]

Author: auria Point 1 done in r10371.

[supertuxkart-sourceforge-migration]

Author: auria and point 2 in r10377