Added copyright information / credits dialog into the main menu.
Inspired by [tabascoeye/workadventure](7b7c695f76), requested by Chaoschemnitz
Credits dialog shows a text field. It's content should be provided by map authors
by defining a map property of type 'string' with the unique key 'license'.
Cleanup of the main menu:
- removed outline from button:active
- button paddings/margins
- removed "Create map" and "Share url"
- renamed buttons "Edit name" / "Edit skin" into "Name" / "Avatar" for
better DE support
