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();

src/gui/MixerChannelView.cpp

@@ -104,7 +104,7 @@ namespace lmms::gui
         m_muteButton->setInactiveGraphic(embed::getIconPixmap("led_green"));
         m_muteButton->setCheckable(true);
         m_muteButton->setToolTip(tr("Mute this channel"));
-        connect(&mixerChannel->m_muteModel, &BoolModel::dataChanged, mixerView, &MixerView::toggledMute, Qt::DirectConnection);
+        connect(&mixerChannel->m_muteModel, &BoolModel::dataChanged, this, &MixerChannelView::toggledMute, Qt::DirectConnection);
 
         m_soloButton = new PixmapButton(this, tr("Solo"));
         m_soloButton->setModel(&mixerChannel->m_soloModel);
@@ -274,11 +274,24 @@ namespace lmms::gui
 
     void MixerChannelView::setChannelIndex(int index)
     {
+        // First disconnect the signals of the previous mixer channel
+        auto * currentMixerChannel = Engine::mixer()->mixerChannel(m_channelIndex);
+        disconnect(&currentMixerChannel->m_muteModel, &BoolModel::dataChanged, this, &MixerChannelView::toggledMute);
+        disconnect(&currentMixerChannel->m_soloModel, &BoolModel::dataChanged, m_mixerView, &MixerView::toggledSolo);
+
         MixerChannel* mixerChannel = Engine::mixer()->mixerChannel(index);
         m_fader->setModel(&mixerChannel->m_volumeModel);
-        m_muteButton->setModel(&mixerChannel->m_muteModel);
-        m_soloButton->setModel(&mixerChannel->m_soloModel);
+
+        auto * muteModel = &mixerChannel->m_muteModel;
+        m_muteButton->setModel(muteModel);
+        connect(muteModel, &BoolModel::dataChanged, this, &MixerChannelView::toggledMute, Qt::DirectConnection);
+
+        auto * soloModel = &mixerChannel->m_soloModel;
+        m_soloButton->setModel(soloModel);
+        connect(soloModel, &BoolModel::dataChanged, m_mixerView, &MixerView::toggledSolo, Qt::DirectConnection);
+
         m_effectRackView->setModel(&mixerChannel->m_fxChain);
+
         m_channelIndex = index;
     }
 
@@ -437,6 +450,11 @@ namespace lmms::gui
         mix->moveChannelRight(m_channelIndex);
     }
 
+    void MixerChannelView::toggledMute()
+    {
+        update();
+    }
+
     QString MixerChannelView::elideName(const QString& name)
     {
         const auto maxTextHeight = m_renameLineEdit->width();

src/gui/MixerView.cpp

@@ -288,11 +288,6 @@ void MixerView::toggledSolo()
 }
 
 
-void MixerView::toggledMute()
-{
-	updateAllMixerChannels();
-}
-
 void MixerView::updateAllMixerChannels()
 {
 	for (int i = 0; i < m_mixerChannelViews.size(); ++i)