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* docs: remove outdated docs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

docs/latest/advanced/extensions.md

@@ -140,8 +140,8 @@ children: PropTypes.node,
 customProps: PropTypes.object
 ```
 
-Viewport components are managed by the `LayoutManager`. Which Viewport component
-is used depends on:
+Viewport components are managed by the `ViewportGrid` Component. Which Viewport
+component is used depends on:
 
 - The Layout Configuration
 - Registered SopClassHandlers
@@ -159,8 +159,8 @@ For a complete example implementation,
 An extension can register a Toolbar Module by providing a `getToolbarModule()`
 method that returns a React Component. The component does not receive any props.
 If you want to modify or react to state, you will need to connect to the redux
-store.
-The given toolbar must determine its set of elements and the context of them. The set of elements will be listed on toolbar `definitions`.
+store. The given toolbar must determine its set of elements and the context of
+them. The set of elements will be listed on toolbar `definitions`.
 
 ![Toolbar Extension](../assets/img/extensions-toolbar.gif)
 
@@ -173,7 +173,11 @@ For a complete example implementation,
 
 ##### Toolbar Custom Component
 
-Toolbar elements can define its own custom react component to be consumed when rendering it. So far, it accepts `Functional` and `Class` Components. For that, you just need to expose your `CustomToolbarComponent` as the value of key `CustomComponent`. In case the property `CustomComponent` is not present, a default toolbar component will be used to render it. See bellow
+Toolbar elements can define its own custom react component to be consumed when
+rendering it. So far, it accepts `Functional` and `Class` Components. For that,
+you just need to expose your `CustomToolbarComponent` as the value of key
+`CustomComponent`. In case the property `CustomComponent` is not present, a
+default toolbar component will be used to render it. See bellow
 
 ```js
 definitions: [
@@ -188,8 +192,11 @@ definitions: [
 ]
 
 ```
+
 `CustomComponent` components will receive the following props:
-- parentContext: parent context. (In most of the cases it will be a ToolbarRow instance)
+
+- parentContext: parent context. (In most of the cases it will be a ToolbarRow
+  instance)
 - toolbarClickCallback: callback method when clicking on toolbar
 - button: its own definition object
 - key: react key prop