Add the ability to fade topics in map and Gantt and other views using a SmartRule

While it is possible to create an effect resembling a fade by choosing muted fill, line and font colours for topics which can be applied via a Smartrule, it would much simpler to be able to apply a "proper" fade as a single effect (this is another suggestion based on this discussion: https://community.mindmanager.com/topic/1810-fade-topics-in-gantt-pro)

Not only would this be easier, it would also fade icons, coloured tags and topic info display background colours, none of which can currently be faded as an effect in a SmartRule.

It may also be useful to hide topics completely as a SmartRule effect, but I'm still thinking about the implications of this.