gfile win32 unicode fixes and refactoring

Submitted by Adrian Johnson `@ajohnson`

Assigned to poppler-bugs

The aim of the following patches is to:

Improve win32 unicode support in goo/gfile.cc.
Eliminate duplicated functions that differ only by char*/wchar_t*. All internal functions should use char* and the win32 wchar_t conversion should be performed at the point where the win32 API is invoked.
Eliminate unused code.
Eventually get windows.h out of the header files. It slows down the compile and pollutes the namespace.
Plus some other mingw fixes and build cleanups I found while working on this.

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