Most of harfbuzz clients don't need Uniscribe and DirectWrite support at all
as harfbuzz is a replacement for Uniscribe so Chromium and Firefox are using
harfbuzz without them on Windows.

This removal also helps win32 nmake clients to have reduced binary size and
libs count.

Add into the NMake Makefiles to build the DirectWrite shaping backend,
but as PR #134 mentions, this is considered to be in an experimental state,
so don't include this in the build by default for now.  This is most probably
going to replace the Uniscribe backend eventually, since DirectWrite is meant
to be Uniscribe's replacement, and is needed for Windows Store apps if a
system shaping API is to be used.

This adds a set of NMake Makefiles that can be used to build HarfBuzz, from
the standard basic build building the minimal HarfBuzz DLL (consisting
of OpenType, fallback and Uniscribe support only), to a full fledged build
consisting of GLib and FreeType support, as well as building the utilities,
the test programs in src/ and test/api, and HarfBuzz-ICU and
HarfBuzz-GObject, and up to building the introspection files.  This means a
flexible build mechanism is supported here, so anything that is supported
for a Windows build (code-wise), should all be supported by this build
system.

As in an earlier commit, the source listings are shared with the autotools
builds with the various Makefile.sources in src/, src/hb-ucdn and util/, and
this set of NMake Makefiles will transform these lists into the form they
want.

In the current form, all the test programs in test/api pass, and this has
been checked successfully with 'make -j8 distcheck'.