Permalink in URL

[joto]

The URL should reflect all the major state of the application to allow linking into it. Currently it only contains the location of the map.

At least the following information should be reflected in the URL:

• [x] location of map
• [x] chosen background layer
• [x] chosen overlay layer
• [x] setting of opacity slider
• [ ] selection of feature(s)
• [x] open tab