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* docs: remove outdated docs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/viewer/src/connectedComponents/PluginSwitch.js

@@ -13,9 +13,7 @@ class PluginSwitch extends Component {
   render() {
     return (
       <div className="PluginSwitch">
-        <ToolbarButton       label = "2D MPR"
-                             icon = "cube"
-                             onClick = {this.props.mpr} />
+        <ToolbarButton label="2D MPR" icon="cube" onClick={this.props.mpr} />
       </div>
     );
   }