Fix mixer channel updates on solo/mute

Fix the update of the mixer channels whenever a channel is soloed or muted.

The solution is rather "brutal" as it updates all mixer channel views when one of the models changes.

Introduce private helper method `MixerView::updateAllMixerChannels`. It calls the `update` method on each `MixerChannelView` so that they can get repainted.

Call `updateAllMixerChannels` at the end of the existing method `toggledSolo`.

Introduce a new method `MixerView::toggledMute` which is called whenever the mute model of a channel changes. It also updates all mixer channels.

Fixes #7054.

include/MixerView.h

@@ -98,6 +98,10 @@ protected:
 private slots:
 	void updateFaders();
 	void toggledSolo();
+	void toggledMute();
+
+private:
+	void updateAllMixerChannels();
 
 private:
 	QVector<MixerChannelView*> m_mixerChannelViews;