Add GUI

## Requirements
- [x] ~~Mondo GUI~~ edit: switch to feature tree
- [x] EULA (same as CastFORM README)
- [x] Architecture check (Flutter for Windows is 64-bit only) - 25df5c7
    - [x] Check win 10+ - 25df5c7
- [x] Full vs Custom install (i.e. vcredist optional)
- [x] Completion dialog message
- [x] Test
- [ ] Refine artwork