Warn better about the risks of executing Python code from projects

[rouault]

Related to "social experiment" https://lists.osgeo.org/pipermail/qgis-user/2019-October/044155.html

The notification when opening a project that contains macro doesn't warn enough about the potential risks. I'd suggest that the "Enable macro" link opens a message box explaining more in detail what could happen if the code was hostile. Bonus point would be a way to inspect the code before giving the consent or not.

Apparently from @elpaso feedback, same issue with code in forms

[Pedro-Murteira]

IMHO this ticket should perhaps be labelled as a feature request, the report mentioned from the users mailing list is referring to a crash which is in fact a bug, be this ticket is more like a enhancement.