Multi users

Several users should be able to use Muallim.
There are two ways users can be authenticated:
* Display a login window so users could choose their account -> users should create an account, but any number of users could then use Muallim
* Use the linux account of the user -> less things to implement but each user who wants to use Muallim should have a linux account

When a user is authenticated he should be able to continue the teaching where he left last time.