Add filter/search box api support to custom tree views

[eamodio]

It would be great for an extension to provide the same filter/search box support that the new Outline view supports. Searching and filter custom views is a request I'm hearing more and more with GitLens.

/cc @jrieken

[Gruntfuggly]

This would be great, but it doesn't necessarily need to just be filter/search. There are other inputs which would be usefully located in the tree view - renaming, for example, so it would be cool if the API can be fairly generic...

[ntotten]

This would be helpful for us on the salesforce test explorer as well. https://github.com/forcedotcom/salesforcedx-vscode/issues/605

I agree the functionality should be generic. Allow extensions to add custom UI to the view's toolbar.