CMake build system

[alexey-lysiuk]

How about replacing qmake based build system with CMake one?

Pros:

• No need to install Qt for users of other IDEs (i.e. Visual Studio or Xcode) and desktop environments (other than KDE)
• Better support for dependency lookup, no more hard-coded SDL2 paths
• One project/solution for all targets (probably this is no longer an issue, I didn't use Qt for a while)
• QtCreator can still be used
• Travis CI and AppVeyor supports it out of the box

Both build systems can coexists if needed.

I will create CMake configuration file(s), just let me know if you are OK with it.

[Panzerschrek]

Hi. Good idea. I wanted to do it myself, but I did not have enough time for this. You can add CMake files, but do not remove qmake files. Also, do not forget about utilities.