Some Windows support related changes

 - Increased buffer size of AVIOContext in AVQt::Demuxer
 - Fixed bug in AVQt::Demuxer::init(): Allocation has to be made with av_malloc()
 - Removed include of experimental C++ headers in Muxer.cpp (Not required any more)
 - Fixed thread safe calls of GUI related functions in AVQt::OpenGLRenderer
 - Added Windows related sections to CMake files (Passing Libav and OpenAL locations with commandline arguments instead of hardcoding them)
 - Adapted example main.cpp to current state of Windows support