Restrict custom recipe properties to upper case

[jkloetzke]

For custom plugin settings in default.yaml the names must not start with a lower case letter. This should have been the case for custom recipe properties too but apparently I missed to implement that. The question is, if this is still rectifiable.

@rhubert @mahaase @sbixl @ThomasFeher would that break existing setups?

The background is that such plugins might break with future Bob versions if the custom recipe property collides with a built-in one. By enforcing a strict namespace separation, this cannot happen.

[rhubert]

I'm not aware of any plugins using custom properties on our side - go ahead.

[sbixl]

We are not yet using these features, so no problems are to be expected.

[ThomasFeher]

We are not using any custom settings.