feat: expose some app internals as window.app (#1735)

* feat: expose some app internals as window.app

This can help developers explore and access some
internal functionaltiy for debugging in the console.

For example, this command can download the currently
viewed study:

ohif.app.commandsManager.runCommand("downloadAndZip", {listOfUIDs: [window.location.href.split("/").pop()]})

TODO: collect this example and other handy functions on a wiki page

Co-authored-by: dannyrb <danny.ri.brown@gmail.com>

* fix: typo in window.ohif.app assignment

Co-authored-by: dannyrb <danny.ri.brown@gmail.com>

platform/viewer/src/App.js

@@ -71,6 +71,14 @@ let extensionManager;
 // TODO[react] Use a provider when the whole tree is React
 window.store = store;
 
+window.ohif = window.ohif || {};
+window.ohif.app = {
+  commandsManager,
+  hotkeysManager,
+  servicesManager,
+  extensionManager,
+};
+
 class App extends Component {
   static propTypes = {
     config: PropTypes.oneOfType([