Implement TestBench4 features in debug window (#12694)

Implement TestBench4 features in debug window (#12694)

Done:
- VaadinFinderLocatorStrategy returns optimal paths for any Widget
selectable by it.
- TestBenchSection of the Debug Window picks and clears as expected.
- Debug Window no longer presents user with a widget hierarchy
- Translation from VaadinFinderLocatorStrategy query strings to
ElementQuery API calls (should be done in SelectorPath.java)
- Make SelectorPaths change background color when hovered

Change-Id: Ie122f962a319ddf560fa9ac4f6bc57f32a120f91