feat(screenshare): emit source type when starting screenshare (#3959)

* feat(screenshare): emit source type when starting screenshare

* squash: update doc

conference.js

         this.isSharingScreen = newStream && newStream.videoType === 'desktop';
 
         if (wasSharingScreen !== this.isSharingScreen) {
-            APP.API.notifyScreenSharingStatusChanged(this.isSharingScreen);
+            const details = {};
+
+            if (this.isSharingScreen) {
+                details.sourceType = newStream.sourceType;
+            }
+
+            APP.API.notifyScreenSharingStatusChanged(
+                this.isSharingScreen, details);
         }
     },
 

doc/api.md

 * **screenSharingStatusChanged** - receives event notifications about turning on/off the local user screen sharing. The listener will receive object with the following structure:
 ```javascript
 {
-"on": on //whether screen sharing is on
+"on": on, //whether screen sharing is on
+"details": {
+
+    // From where the screen sharing is capturing, if known. Values which are
+    // passed include "window", "screen", "proxy", "device". The value undefined
+    // will be passed if the source type is unknown or screen share is off.
+    sourceType: sourceType
+}
 }
 ```
 

modules/API/API.js

      * has been turned on/off.
      *
      * @param {boolean} on - True if screen sharing is enabled.
+     * @param {Object} details - Additional information about the screen
+     * sharing.
+     * @param {string} details.sourceType - Type of device or window the screen
+     * share is capturing.
      * @returns {void}
      */
-    notifyScreenSharingStatusChanged(on: boolean) {
+    notifyScreenSharingStatusChanged(on: boolean, details: Object) {
         this._sendEvent({
             name: 'screen-sharing-status-changed',
-            on
+            on,
+            details
         });
     }