Separate binaries for GUI and console on Windows

Drawbacks:

* Compile time is twice as long.
* This creates two binaries with similar size (console app is just missing the icon).

Advantages:

* Basically just: no code needs to be changed.

Any critic is welcome :).

EDIT: I've solved the binary size and compilation time with the other main() in src/platform/main.cpp (as mentioned in comments).