CMake improvements for Windows / Visual Studio

[Harteex]

• Since autodetection of libraries doesn't work that good on Windows, some documentation is probably needed, especially for the cmake variable SDL2_LIBRARY, which you might think you need to fill yourself.
• When running the compiled application, a console window pop ups next to the ExLauncher window. This is fixed by going into project properties in visual studio -> Linker -> System and setting SubSystem from Console to Windows. A fix which does this already in CMake would be great, if possible.
• The generated projects ALL_BUILD and ZERO_CHECK doesn't seem necessary.
• It would be nice if resources and include files are included in the project, not just source files.

[Harteex]

pass --subsystem=windows (GCC) /SUBSYSTEM:WINDOWS (MSVC) to the linker