Choicelist Edit element change display

* Element need to be selected in treeview to be visible in layout mode
Widget icon prefix change

icon->wegas-icon
Merge branch 'master' of github.com:Heigvd/Wegas

# By Francois-Xavier Aeberhard (2) and Anthony Geiser (1)
# Via Anthony Geiser (2) and Francois-Xavier Aeberhard (2)
* 'master' of github.com:Heigvd/Wegas:
  Add icons in variable treeview
  Fix #243
  Fix focus