Instruct CMake to create a Windows GUI application

Keep the entry point at main(), though, for simplicity.
Add proper support for running CopyQ via command line and GUI on Windows

Files are taken as-is from the "dualsubsystem" project on Google Code
(tester.com was just renamed to copyq.com), see

    http://code.google.com/p/dualsubsystem/

For some more background information and alternative techniques, see:

    http://stackoverflow.com/a/494000/1127485
DualModeI: Add a missing newline at end of file
DualModeI: Use more secure file I/O functions