Refactor ClientUser to UserModel/MainWindow signal & slot connections.

Previously the muteDeafChanged() and the talkingChanged() message were
used to indicate user state changes to model and GUI. With time even
changes not related to mute and deafen started to use muteDeafChanged()
to trigger wanted GUI update behavior that was uniform for all state changes.
This re-use muddied the meaning of the corresponding signals and slots.

This patch refactors the corresponding signals & slots to have well defined
meanings again. The ClientUser gains a prioritySpeakerStateChanged and a
recordingStateChanged signal. The existing state change signals were renamed
accordingly. As the upper layers - as indicated by code duplication in the
model as well as signal & slot re-use - don't need that granularity the
corresponding slots were unified into a userStateChanged() slots all
xxxStateChanged() functions are connected to.