Check if a variable used in a control is declared

[romaintailhurat]

A lot of error happening when generating a questionnaire are due to missing variables in control expressions.

In order to smooth the process, we will implement a global check in Pogues :

1. when a designer click on the "Validate" button of a sequence or question
2. when a variable is deleted
   a. when a question is deleted (thus the variable(s) it contains)
   b. when a collected, calculated or external variable is deleted

The resulting error should be displayed in the questionnaire structure view, as it is done for example for structural problems (eg a questionnaire without any sequence)