Made it compile on windows

zlib is needed in 3rd-party folder (added to repo)
Make DiskWriter do the uncompressing instead

I had very slow decompression on windows, but it improved by increasing
the buffer size. It made more sense to make DiskWriter responsible for
decompression  (and only supply it with a image file name). Had to make
it a QObject to get the progress bar working.

Also updated zlib1.dll with a more optimized one (haven't done any
profiling, but docs say it is).

Also some general cleanup with enabled/disabled widgets.

No cancel button yet, though
Extract image file names from url

Also change xml file name to sf_data.xml
Divide DiskWriter implementations in _windows and _unix classes