Edit widget after adding

*Fix #196
missing correction due to previous datasource refactor
Merge branch 'master' of github.com:Heigvd/Wegas

# By Francois-Xavier Aeberhard
# Via Francois-Xavier Aeberhard
* 'master' of github.com:Heigvd/Wegas:
  Fix serialization issue in variableinstance
  Fix issue in scope -> instance update