Feature request: Transparent visited places

[JudithBirke]

Current state: Visited places are in a specific colour.

Target state: Visited places can be made transparent.

Sample application 1: The background image contains objects like badges, flowers, trophies, smilies etc. which are covered by places that have not been visited yet. Once a place has been visited the corresponding object becomes visible in the node.

Sample application 2: The places to be visited stand for problems that need to be solved by users. As users go along solving problems, the background image becomes more and more visible to them.