Improve additional_install_files for debug/release

This improves dependency copying and adds separated debug/release
install files.
Allow copying of all dependencies (windows only)

If the cmake user variable COPY_DEPENDENCIES is set, this script will
make it so that a windows build will automatically copy all required
dependencies (FFmpeg, x264, and Qt5) to the respective
additional_install_files\exec(32|64) directory.  This makes it much
easier to set up a development environment on windows, and much easier
to make usable test builds.

It will also copy the appropriate Direct3D compiler DLL, along with
dependencies of dependencies (the icu*.dll and EGL/GLES files for Qt)
Add support for 32/64bit environment variables

This can make it a bit easier to set up 32bit/64bit environments
separately on windows.  This simply allows you to additionally set
FFmpegPath32, FFmpegPath64, x264Path32, x264Path64, etc.
Update INSTALL instructions for windows/mac