This patch allows us to use our own bundled TWAINDSM.dll (from
twain-dsm project, http://www.twain.org, https://github.com/twain/twain-dsm)
on 64-bit Windows systems that don't have their own TWAIN DLL,
and without requiring users to install it from some other source.
The DLL is put into INSTALLOCATION/program along soffice.bin.
Of course, TWAIN sources still need to be 64-bit themselves (i.e.,
true 64-bit TWAIN drivers required) to be usable by 64-bit program.

Also this drops external/twain/inc/twain/twain.h, which is an old
version copied from twain-dsm, and instead uses the tarball to get
the header (required by Library_scn on both 32- and 64-bit Windows).

Change-Id: I191027f3221ce46db17f50db91c9cb9315900810
Reviewed-on: https://gerrit.libreoffice.org/46992Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>