When the iFrame api is used to set a preferred audio output using
options passed into the JitsiMeetExternalAPI constructor, no logic
fires to actually change the audio output destination.

* 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

BaseApp does all the heavy-lifting related to creating the redux store,
navigation, and so on.

App currently handles URL props and actually triggering navigation based on
them.