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* docs: remove outdated docs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/index.js

@@ -1,8 +1,4 @@
-import {
-  ExampleDropTarget,
-  StudyBrowser,
-  ThumbnailEntry,
-} from './studyBrowser';
+import { StudyBrowser, ThumbnailEntry } from './studyBrowser';
 import { LayoutButton, LayoutChooser } from './layoutButton';
 import { MeasurementTable, MeasurementTableItem } from './measurementTable';
 import { Overlay, OverlayTrigger } from './overlayTrigger';
@@ -26,7 +22,6 @@ import { Tooltip } from './tooltip';
 export {
   Checkbox,
   CineDialog,
-  ExampleDropTarget,
   LayoutButton,
   LayoutChooser,
   MeasurementTable,