If the back is getting an error (because the user has no right to set a variable),
instead of failing directly, let's try to reload the map (maybe we have cached a wrong version of the map).
Up until this commit, variables nested in object layers inside group layers where not found by the front nor the back.
This PR changes analysis so that variables can be detected.
The "persist" property was not taken into account and all variables were stored in DB. This change makes sure only variables tagged with "persist" are actually persisted.
Setting a variable would makes the application enter in an infinite loop of events (between all the scripts and the back)
This fix makes sure a variable does not emit any event if it is changed to a value it already has.
