feat(welcome-page): be able to open settings dialog (#3327)

* feat(welcome-page): be able to open settings dialog

- Create a getter for getting a settings tab's props so the device
  selection tab can get updated available devices.
- Be able to call a function from a tab after it has mounted. This is
  used for device selection to essentially call enumerateDevices on
  the welcome page so the device selectors are populated.
- Remove event UIEvents.AUDIO_OUTPUT_DEVICE_CHANGED. Instead directly call
  setAudioOutputDeviceId where possible.
- Fix initialization of the audioOutputDeviceId in settings by defaulting
  the audio output device to the one set in settings.

* squash: updateAvailableDevices -> getAvailableDevices, add comment for propsUpdateFunction

react/features/base/settings/reducer.js

@@ -138,7 +138,7 @@ function _initSettings(featureState) {
         if (settings.audioOutputDeviceId
             !== JitsiMeetJS.mediaDevices.getAudioOutputDevice()) {
             JitsiMeetJS.mediaDevices.setAudioOutputDevice(
-                audioOutputDeviceId
+                settings.audioOutputDeviceId
             ).catch(ex => {
                 logger.warn('Failed to set audio output device from local '
                     + 'storage. Default audio output device will be used'