Read-only variables for reduced code generation

[PhonicUK]

When a variable is only used on a control which is not editable and will never be changed by the UI (such as a label) - there's no need for the generated code to produce a stub for a setter.

It would be nice if we could explicitly set variables as read only so that the code generation won't produce set_var_my_variable(value) stubs that get implemented as nothing and serve no purpose.

Right now I'm either implementing these as empty methods where I don't want/need them, or manually removing the stubs - but this is extra work. Removing these would help make the overall code slightly cleaner by reducing unnecessary cruft.