feat(external_api) facilitate gDM Electron

In order to use gDM in Electron the flow is somewhat reversed. It starts
from the Electron main process, so we need an API in the external_api
that can trigger the builtin picker. The picker is still necessary.

modules/API/API.js

@@ -75,6 +75,7 @@ import {
     toggleChat
 } from '../../react/features/chat/actions';
 import { openChat } from '../../react/features/chat/actions.web';
+import { showDesktopPicker } from '../../react/features/desktop-picker/actions';
 import {
     processExternalDeviceRequest
 } from '../../react/features/device-selection/functions';
@@ -1048,6 +1049,23 @@ function initCommands() {
         }
         case '_new_electron_screensharing_supported': {
             callback(true);
+
+            break;
+        }
+        case 'open-desktop-picker': {
+            const { desktopSharingSources } = APP.store.getState()['features/base/config'];
+            const options = {
+                desktopSharingSources: desktopSharingSources ?? [ 'screen', 'window' ]
+            };
+            const onSourceChoose = (_streamId, _type, screenShareAudio, source) => {
+                callback({
+                    screenShareAudio,
+                    source
+                });
+            };
+
+            dispatch(showDesktopPicker(options, onSourceChoose));
+
             break;
         }
         default: