Make KSharedDataCache work for Windows

This removes the dummy windows implementation and uses the standard KSharedDataCache implementation by mapping mmap API to Windows API, which is a common pattern for that case. The copyright header in mmap_win_p.h clarifies where this code was obtained. It was slightly modified by me to provide everything the KSharedDataCache API needed.

As an immediate benefit to most applications it enables the icon cache.

I have only tested this with kf5 and Okular and Kleopatra as I don't have other Windows builds at hand.