If there are several separate conditional format elements that can be
represented as just one (with several ranges), try to do that.

A particular customer document used to take 3 minutes 20 seconds to
load, and it contained so many (tens of thousands) conditional formats
that the Format> Conditional Formatting> Manage... dialog was
practically impossible to use.

Now loading that document takes 15 seconds and there are just a
handful of separate conditional formats.

Also add a simple unit test to verify the deduplication.

Change-Id: I7c468af99956d4646ee5507390f1476caff52325
Reviewed-on: https://gerrit.libreoffice.org/45460Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>