fix(config): Document desktopSharingSources and improve types

config.js

     //         video: true
     //     },
     // },
+    // The default type of desktop sharing sources that will be used in the electron app.
+    // desktopSharingSources: ['screen', 'window'],
+
     // Disables the echo cancelation for local audio tracks.
     // disableAEC: true,
 

react/features/base/config/configType.ts

 import { ToolbarButton } from '../../toolbox/types';
 import { ILoggingConfig } from '../logging/types';
+import { DesktopSharingSourceType } from '../tracks/types';
 
 type ButtonsWithNotifyClick = 'camera' |
     'chat' |
         max?: number;
         min?: number;
     };
+    desktopSharingSources?: Array<DesktopSharingSourceType>;
     dialInConfCodeUrl?: string;
     dialInNumbersUrl?: string;
     dialOutAuthUrl?: string;

react/features/base/tracks/types.ts

         };
     };
     desktopSharingSourceDevice?: string;
-    desktopSharingSources?: string[];
+    desktopSharingSources?: Array<DesktopSharingSourceType>;
     devices?: string[];
     facingMode?: string;
     micDeviceId?: string | null;
     shareOptions: IShareOptions;
 }
 
+export type DesktopSharingSourceType = 'screen' | 'window';
+
 export interface IShareOptions {
     desktopSharingSourceDevice?: string;
-    desktopSharingSources?: string[];
+    desktopSharingSources?: Array<DesktopSharingSourceType>;
     desktopStream?: any;
 }
 

react/features/desktop-picker/actions.ts

 import { openDialog } from '../base/dialog/actions';
+import { DesktopSharingSourceType } from '../base/tracks/types';
 
 import DesktopPicker from './components/DesktopPicker';
 
+type Options = {
+    desktopSharingSources?: Array<DesktopSharingSourceType>;
+};
+
 /**
  * Signals to open a dialog with the DesktopPicker component.
  *
  * a DesktopCapturerSource has been chosen.
  * @returns {Object}
  */
-export function showDesktopPicker(options: { desktopSharingSources?: any; } = {}, onSourceChoose: Function) {
+export function showDesktopPicker(options: Options = {}, onSourceChoose: Function) {
     const { desktopSharingSources } = options;
 
     return openDialog(DesktopPicker, {

react/features/desktop-picker/components/DesktopPicker.tsx

 import { IStore } from '../../app/types';
 import { hideDialog } from '../../base/dialog/actions';
 import { translate } from '../../base/i18n/functions';
+import { DesktopSharingSourceType } from '../../base/tracks/types';
 import Dialog from '../../base/ui/components/web/Dialog';
 import Tabs from '../../base/ui/components/web/Tabs';
 import { THUMBNAIL_SIZE } from '../constants';
     window: 'dialog.applicationWindow'
 };
 
-const VALID_TYPES = Object.keys(TAB_LABELS);
+const VALID_TYPES = Object.keys(TAB_LABELS) as DesktopSharingSourceType[];
 
 /**
  * The type of the React {@code Component} props of {@link DesktopPicker}.
     /**
      * An array with desktop sharing sources to be displayed.
      */
-    desktopSharingSources: Array<string>;
+    desktopSharingSources: Array<DesktopSharingSourceType>;
 
     /**
      * Used to request DesktopCapturerSources.
     /**
      * The desktop source types to fetch previews for.
      */
-    types: Array<string>;
+    types: Array<DesktopSharingSourceType>;
 }
 
 
      * @private
      * @returns {Array<string>} The filtered types.
      */
-    static _getValidTypes(types: string[] = []) {
+    static _getValidTypes(types: DesktopSharingSourceType[] = []) {
         return types.filter(
             type => VALID_TYPES.includes(type));
     }
             = types.map(
                 type => {
                     return {
-                        accessibilityLabel: t(TAB_LABELS[type as keyof typeof TAB_LABELS]),
+                        accessibilityLabel: t(TAB_LABELS[type]),
                         id: `${type}`,
                         controlsId: `${type}-panel`,
-                        label: t(TAB_LABELS[type as keyof typeof TAB_LABELS])
+                        label: t(TAB_LABELS[type])
                     };
                 });