Currently the following are implemented:

- AudioMuteButton
- HangupButton
- VideoMuteButton

In order to implement these new buttons a new abstract class was introduced,
which abstracts the ToolboxItem into a button with enough hooks so a stateful
and a stateless version of it can be created.

This patch only adds the stateful implementation of the aforementioned buttons.

* ref(toolbar): add accessibility labels for torture tests

* squash: update propTypes

The redux flows for video muting may not cover all cases
that the conference.js flows cover. Just exercise the old
flows to be safe.

The redux flows do not account for as many scenarios, such
as config.startWithAudioMuted being true.