Add support for search functionality in TableView and TreeView

[rcoreilly]

In FileView, not clear if beneficial relative to completion in the textfield.. but probably some general kind of search box with < > arrows that scrolls to different rows in the tableview would be easy and generally useful.

[rcoreilly]

added general search to all layouts just by typing if nothing else is in focus, and completion is probably best for fileview. but might be good to have a demo test case of search in a table view just to work through any of those issues.

[rcoreilly]

Scope: this is strictly within the widget itself. There are other possible issues with top app bar that are related but distinct. Still unclear of we want some kind of specific scoped search within these views.

[kkoreilly]

We may want to add a global search with a text field via Ctrl+F.