Introduce the mediapicker field (#1125)

Added a new `mediapicker` form field which allows to select a media from any page
+ 
Provided an option to control how parent select field displays

themes/grav/app/main.js

@@ -8,6 +8,7 @@ import Forms from './forms';
 import Scrollbar, { Instance as contentScrollbar } from './utils/scrollbar';
 import './plugins';
 import './themes';
+import { Filter as MediaFilter, Instance as MediaFilterInstance} from './media';
 
 // bootstrap jQuery extensions
 import 'bootstrap/js/transition';
@@ -65,5 +66,9 @@ export default {
     Sidebar: {
         Sidebar,
         Instance: sidebar
+    },
+    MediaFilter: {
+        MediaFilter,
+        Instance: MediaFilterInstance
     }
 };