fix(conference): start camera later on

Instead of disabling the video button in the toolbar, mark it as muted,
so that the user can click it to try enable video later on, even if
joined without video (either declined permission or was starting with
screen streaming and dismissed the dialog).

conference.js

                 }
 
                 if (!tracks.find((t) => t.isVideoTrack())) {
-                    APP.UI.setCameraButtonEnabled(false);
+                    // Instead of disabling the button we want to show button
+                    // muted, so that the user can have the opportunity to add
+                    // the video later on, even if joined without it.
+                    this.videoMuted = true;
+                    APP.UI.setVideoMuted(this.getMyUserId(), this.videoMuted);
                 }
 
                 this._initDeviceList();
                     newStream.videoType === 'camera'
                         && APP.UI.setCameraButtonEnabled(true);
                 } else {
-                    this.videoMuted = false;
+                    // No video is treated the same way as being video muted
+                    this.videoMuted = true;
                     this.isSharingScreen = false;
                 }
                 APP.UI.setVideoMuted(this.getMyUserId(), this.videoMuted);
         APP.UI.addListener(UIEvents.VIDEO_MUTED, muted => {
             if (this.isAudioOnly() && !muted) {
                 this._displayAudioOnlyTooltip('videoMute');
-                return;
+            } else if (!localVideo && this.videoMuted && !muted) {
+                // Maybe try to create local video if there wasn't any ?
+                // This handles the case when user joined with no video
+                // (dismissed screen sharing screen), but decided to add it
+                // later on by clicking on muted video icon.
+                createLocalTracks({ devices: ['video'] }, false)
+                    .then(([videoTrack]) => {
+                        APP.conference.useVideoStream(videoTrack);
+                    })
+                    .catch(error => {
+                        APP.UI.showDeviceErrorDialog(null, error);
+                    });
+            } else {
+                muteLocalVideo(muted);
             }
-
-            muteLocalVideo(muted);
         });
 
         room.on(ConnectionQualityEvents.LOCAL_STATS_UPDATED,