Check if a variable used in a control is declared

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)

No files or tests are named. Start by tracing the Validate actions for sequences and questions, along with question and variable deletion flows, then inspect how structural errors appear in the questionnaire structure view. Done means missing variables in control expressions are detected in each listed case and the resulting error is displayed there.

Written by the indexing model from the issue text.