Prepare for builds against ICU 64

l10n
#1

Hello all,

For information ICU 64 adds new scripts and Unicode blocks from Unicode
12 that need to be handled.

This adds 9 new resource strings that will show up for translators, but
are not really necessary to be translated; unless a distribution
actually builds against ICU 64 they won't be displayed and even then
only in very rare circumstances (in Special Characters for fonts
containing the glyphs of these new Unicode blocks, unlikely). TDF builds
will not be affected as they are built against internal ICU 63.

Thanks to Eike, it has been added to 6.3 branch.

Cheers
Sophie