eit: Scrape sub-title from summary in OTA EIT. (#4578).

eit: Scrape sub-title from summary in OTA EIT. (#4578).

The Freeview/Freesat frequently have a subtitle as part of the
summary. So we have "Treehouse of Horror IX: Three scary stories."
from which we can deduce the subtitle as "Treehouse of Horror IX".

Other variants are "...title_continuation. Subtitle" (so the real
title of the program is split in to the summary), and
"x/y. Subtitle" where x/y is the episode number.

So allow scraping of this and use it as the subtitle. If we cannot
scrape a subtitle then we continue the existing practice of using
the summary buffer for the subtitle.

The subtitle is currently NOT removed from the summary.

Issue: #4578