Add filter to show a level title given a number

In the new interface, given a level we display a message:
2: Easy
4: Easy/Medium
8: Medium
16: Medium/Hard
32: Hard
Fix question list when creating new question set

They are now reinitialized correctly when a new question set is
displayed to the user
Merge pull request #21 from mlopez-firstfactory/master

Level filter, fix question list when a new set is created