Add GWin32InputStream and GWin32OutputStream classes

Add GWin32InputStream and GWin32OutputStream classes

Correspond to GUnixInputStream and GUnixOutputStream. No true async
support though. But that is how the Win32 API is, for files not
explicitly opened for so-called overlapped IO.

The API to create these streams takes Win32 HANDLEs. Not file
descriptors, because file descriptors are specific to the C library
used. The user code and GLib might be using different C libraries.

Also add a test program for the new classes, and a gio-windows-2.0.pc
file.