Rewrite settings handling

**Don't merge**

Integrates (rewritten) settings dialog into our window system.

Tasks:
* [x] remove defaultvalues from horizons/engine/settings.py an query them from the settingstemplate
* [x] add settings which aren't mapped yet 
* [x] reintegrate settingsfile upgrade code
* [x] change Hotkey setting on the fly (without restart) Thats works but not for NUMPAD keys. See #2156
* [x] reset changes to default