Enhance "datalayers switcher"

[yohanboniface]

Here is how it looks currently:

A few feedback:

• it should include colors and icons of each datalayer
• it should be in the bottom of the screen, to be less intrusive (maybe merge with new "permanent credit")

[yohanboniface]

A few more questions/remarks:

• should the users be able to chose the position (top/bottom left/right) ?
• should we include caption text when provided ? Otherwise, we may need to work on a "small caption box", because the current one in the full panel is quite intrusive
• I'm in the process to allow choosing which layers would go in this box: some layer be only be for esthetics reasons, or be unfinished…
• should we allow to display some features in the legend ? either because they represent some sort of features one may want to showcase ("those features are the hospitals"), either because as a feature it should be show cased ("this feature is the finish of the race")

[yohanboniface]

Done in #1687 and following PRs