New MapGenerator & GameType structure

This should solve #545

For both, map generators and gametypes there are now URI. Manager for both types are capable of scanning mods for custom map generators/gametypes. 

Map generators may consist of chunk generators (but does not rely on).
Gametypes defines a default mod selection and a default map generator (using URI). Some hooks are also there. Should be extended on need. 

As an example, there is a Light and Shadow game type + map generator.

Some smaller changes are included too (ColumnLayout, Create New World UI rebuild, ...).