Fix crash on toggleable form pages on layout showcase

There are several issues:

• the XML configuration is not ready for demo (need to setup readonly values for initial layout name, for instance)
• some widgets help contain sample EL expressions, that get evaluated at runtime when displaying this help, and are not printed corrrectly on the showcase reference page (same pb than on NXP-15942) nor on Studio help (where EL is not evaluated)
• the reference pages should all be tested to ensure that page does not crash (at least) nor produces ERROR logs