Improve handling of deactivated highlighting when still on the original 'search page'

[ator-dev]

When a user deactivates MMS in a tab, they expect it to stay off - at least while on the same page. For convenience reasons this is not the case, as automatic activation occurs whenever a page with search keywords is loaded (including refreshes).

Perhaps MMS could have an alternate behaviour, like re-enabling only if the current search keywords are different from the last ones in the tab? This will need more consideration to avoid apparent inconsistency and so confusion.

[ator-dev]

For a visual example, this DuckDuckGo page will always reactivate automatically because it has search keywords (mark+my+search -> mark my search)

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Noticed 03-28: many search engines are considered to have performed a navigation (or update the URL) when reflecting user decisions such as opening an image into a larger view. For this reason the current situation is fairly unworkable.

[ator-dev]

Current plan: to have a 'grace period' which ends as soon as a navigation occurs which does not generate the same set of keywords via the URL (or would not, ignoring the stoplist?). Within this period, navigations would not be allowed to restore deactivated highlighting.