Refactored parts of the code for building native with Microsoft Visual Studio 2012

Small changes for the Windows Explorer plugin, too. (See Ultracopier Pull Request for more Info)

I am not quite sure if this is okay for MinGW - PLEASE TEST BEFORE PULLING!
I removed the ordinal values in the .def file, they _should_ not be needed, but I do not know much about MinGW.

Cheers,
blizzard4591