SE 5.3.0 beta 4: String search within a Settings category

I mentioned this in #12087 but it's probably lost. So let's make it stand out.

This is inspired by the way Firefox works. Let the user just type something in the Settings dialog. If the string occurs in the current category, move focus to that string. That is almost certainly not the actual check box, drop down list, spinner value, whatever. It's probably the name of the setting to the left of the thing that sets it. But when I say it moves focus, I mean when you stop typing & hit Tab, focus should move to that setting field. This search is NOT meant to supplant the search box at the top of the Settings dialog. That searches across categories. What I'm proposing is just a Find in Page (to use Firefox terminology) in the current category. Then F3 could do a Find Next. Repeated F3 presses would eventually loop around to the top of the current category. The search would be restricted to the current category. Of course, Shift+F3 would navigate in reverse, just like in Firefox.

But if you navigate to another category, F3 should search for the same string again, even though it was in a different category earlier. And this new search should again be restricted to the category currently on display.

Closing Settings should forget this search string.

Now, I'm suggesting the F3 shortcut. That's what Firefox uses. We've had plenty of complaints about shortcuts usurping system functions. Plus maybe the user has already assigned F3 to a different shortcut. I suggest again, as I have a number of times, that the Shortcuts dialog should make the Active In column a user modifiable field (#13102, #13373). That would take care of any issues of shortcut collisions. The user could define his own collision resolution without you coding special ad hoc responses to each instance of a user not liking the context in which shortcuts fire. Let the user bear the burden of organizing it all.