Create standalone components for each feature and move all state to them.
Toolbars are now dummy containers.

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.