refactor: layout manager to viewer (#1026)

* docs: formatting

* Prefer numRows and numColumns to percentage width/height for layout

* fix: LayoutManager --> ViewportGrid (get out of UI component library)

* docs: remove outdated docs

* Don't expose ExampleDropTarget

* Revert thumnail entry drag source removal

* Update screaming tests

* fix drag-n-drop

* fix vtk mpr2d CC: @jamesapetts

* remove setSingleLayoutData

* remove vtk qualifier

platform/ui/src/components/studyBrowser/index.js

@@ -1,3 +1,2 @@
-export { ExampleDropTarget } from './ExampleDropTarget.js';
 export { StudyBrowser } from './StudyBrowser.js';
 export { ThumbnailEntry } from './ThumbnailEntry.js';