CMake Rewrite

Most obvious change of this is of course the mostly full rewrite of the CMake build system, which is now able to build and package on every supported platform.

Some other changes this introduces are a few minor fixes to the new Qt code, which makes it compile with non-MSVC compilers.

Also, the reason for the high amount of changed files is that i had to introduce new EXPORT macros to be able to propperly alter between dllimport and dllexport, specialy with the quite complicated libobs-opengl/d3d situation, which requires some functions from the libobs header to still be exported.
The Visual Studio Projects were be modified so they propperly define the LIBOBS_GS_EXPORTS, in the opengl/d3d projects, and LIBOBS_EXPORTS, in libobs, defines.