Ongoing improvements

[avindra]

Next:

• Decouple core game code from Qt
• Tests
• Harden impossible mode: can be beaten still in 1 specific way :^)

Nice to have:

• Continuous integration
• Backport build to Windows^†

Notes

^† The project was bootstrapped with and for Visual Studio 2008, and the story on MS build compatibility is unknown to me. So the build got moved to CMake which is commonly used with Qt projects.