User identification and cross-device gaming

[CrsiX]

Current idea: a user is identified by username and password, so that playing on multiple devices works seamlessly (not at the same time for obvious reasons, but handling one round on a desktop and handling the other on a mobile device should work).

[touhidurrr]

Already implemented, see: yairm210/Unciv#8716 yairm210/Unciv#8770 yairm210/Unciv#8779 yairm210/Unciv#8790