Improve mixer channel update for mute events

Improve the mixer channel update for mute events by not delegating to `MixerView` like for the solo case. Instead the `MixerChannelView` now has its own `toggledMute` slot which simply updates the widget on changes to the mute model.

Also fix `MixerChannelView::setChannelIndex` by disconnecting from the signals of the previous mixer channel and connecting to the signals for the new one.

Remove `toggledMute` from `MixerView`.

The solo implementation is kept as is because it also triggers changes to the core model. So the chain seems to be:
* Solo button clicked in mixer channel view
* Button changes solo model
* Solo model signals to mixer view which was connected via the mixer channel view
* Mixer view performs changes in the other core models
* All mixer channels are updated.

Are better chain would first update the core models and then update the GUI from the changes:
* Solo button clicked in mixer channel view
* Button changes solo model
* Solo model signals to core mixer, i.e. not a view!
* Mixer view performs changes in the other core models
* Changed models emit signal to GUI elements

include/MixerChannelView.h

@@ -97,6 +97,7 @@ namespace lmms::gui
         void removeUnusedChannels();
         void moveChannelLeft();
         void moveChannelRight();
+	    void toggledMute();
 
     private:
         QString elideName(const QString& name);

include/MixerView.h

@@ -98,7 +98,6 @@ protected:
 private slots:
 	void updateFaders();
 	void toggledSolo();
-	void toggledMute();
 
 private:
 	void updateAllMixerChannels();