feat: 🎸 React custom component on toolbar button (#935)

* feat: 🎸 React custom component on toolbar button

Toolbar button can consume/render react custom components. Its only
required to expose the component into toolbar definitions
(toolbarModule)

Closes: 924

* feat: 🎸 Core review. Minor changes on documentation

Closes: 924

* feat: 🎸 Code review

* feat: 🎸 Code review. Changing function call to variable decl

docs/latest/advanced/extensions.md

@@ -160,6 +160,7 @@ 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`.
 
 ![Toolbar Extension](../assets/img/extensions-toolbar.gif)
 
@@ -170,6 +171,31 @@ Toolbar components are rendered in the `ToolbarRow` component.
 For a complete example implementation,
 [check out the OHIFCornerstoneViewport's Toolbar Module](https://github.com/OHIF/Viewers/blob/master/extensions/cornerstone/src/toolbarModule.js).
 
+##### 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
+
+```js
+definitions: [
+...
+    {
+        id: 'Custom',
+        label: 'Custom',
+        icon: 'custom-icon',
+        CustomComponent: CustomToolbarComponent,
+    }
+...
+]
+
+```
+`CustomComponent` components will receive the following props:
+- 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
+- activeButtons: list of active elements
+- isActive: if current
+
 #### SopClassHandler
 
 ...