Fix bug #7112 (Out-of-scope files added to Find in Files results)

Fix bug #7112 -- Apply all the same filtering when updating incrementally as we do when performing the initial search.  Consolidate many of these checks in a new _inSearchScope() and rename the old _inScope() to sound less confusingly similar.

I added a unit test for _one_ case of #7112, but since there aren't tests in general for the incremental results updating it would have been tricky to add lots more.  The [Unit tests for existing find features](https://trello.com/c/lBKh0WXZ/1077-unit-tests-for-existing-find-replace-find-in-files-features) card is tracking writing more tests for this area.

_Also:_ tweak Dialog to allow closing with Ctrl+Enter when focus is in a textarea (where regular Enter is prevented from closing). Helps filter editing.