Allow a custom 'not_enabled' css styling

[rjwills28]

Would it be possible to add a property for facilities to customize the look of 'not_enabled'/'no_write' widgets?

At the moment, the default is to set the opacity to 40% (as per the disabled look in JavaFX).

I opened a ticket a while back showing how this wasn't always ideal - see #3368 discussion and below is an example of where the opacity is set to 40% but gives a strange look on the widget as the green background is coming through:

I explored alternative ways of displaying 'not_enabled' widgets but couldn't find anything that was better and just worked for all widgets.

Adding a property where users can override the default behaviour (i.e. setting the opacity to 40%) seems like a nice alternative. In the case above we could for example keep the opacity at 1.0.