MSVC Builds: Use Custom Build Steps

Use Custom Build Steps for copying config.h from config.h.win32 and for
generating libcroco.def from libcroco.symbols, as it would make it easier
to clean and regenerate if one is cleaning or rebuilding or updateing
those files.

This also puts back into the .sln files the lines that were actually
needed, which were removed from them accidentally from the last commit.