Fix mixer channel updates on solo/mute (#7055)

* 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.

* 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

* Revert "Improve mixer channel update for mute events"

This reverts commit ede65963ea.

* Add comment

After the revert done with commit the code is more consistent again but not in a good way. Hence a comment is added which indicates that an inprovement is needed.

* Abstract mixer retrieval

Abstract the retrieval of the mixer behind the new method `getMixer`. This is done in preparation for some dependency injection so that the `MixerView` does not have to ask the `Engine` for the mixer but gets it injected, a.k.a. the "Hollywood principle": "Don't call us, we'll call you."

It's called `getMixer` and not just mixer because it allows for locale variables to be called `mixer` without confusing it with the method.

* Let MixerView connect directly to models

Let the `MixerView` connect directly to the solo and mute models it is connected with.

Remove the connections that are made in `MixerChannelView` which acted as a proxy which only complicated things.

Add `connectToSoloAndMute` which connects the `MixerView` to the solo and mute models of a given channel. Call it whenever a new channel is created.

Add `disconnectFromSoloAndMute` which disconnects the `MixerView` from the solo and mute models of a given channel. Call it when a channel is deleted.

* Code cleanup

Cleanup code related to the creation of the master channel view.

* Inject the Mixer into the MixerView

Inject the `Mixer` into the `MixerView` via the constructor. This makes it more explicit that the `Mixer` is the model of the view. It also implements the "Dependency Inversion Principle" in that the `MIxerView` does not have to ask the `Engine` for the `Mixer` anymore.

The current changes should be safe in that the `Mixer` instance is static and does not change.

* Fix connections on song load

Disconnect and reconnect in `MixerView::refreshDisplay` which is called when a song is loaded.

include/MixerView.h

@@ -38,6 +38,11 @@
 #include "embed.h"
 #include "EffectRackView.h"
 
+namespace lmms
+{
+	class Mixer;
+}
+
 namespace lmms::gui
 {
 class LMMS_EXPORT MixerView : public QWidget, public ModelView,
@@ -45,7 +50,7 @@ class LMMS_EXPORT MixerView : public QWidget, public ModelView,
 {
 	Q_OBJECT
 public:
-	MixerView();
+	MixerView(Mixer* mixer);
 	void keyPressEvent(QKeyEvent* e) override;
 
 	void saveSettings(QDomDocument& doc, QDomElement& domElement) override;
@@ -97,7 +102,17 @@ protected:
 
 private slots:
 	void updateFaders();
+	// TODO This should be improved. Currently the solo and mute models are connected via
+	// the MixerChannelView's constructor with the MixerView. It would already be an improvement
+	// if the MixerView connected itself to each new MixerChannel that it creates/handles.
 	void toggledSolo();
+	void toggledMute();
+
+private:
+	Mixer* getMixer() const;
+	void updateAllMixerChannels();
+	void connectToSoloAndMute(int channelIndex);
+	void disconnectFromSoloAndMute(int channelIndex);
 
 private:
 	QVector<MixerChannelView*> m_mixerChannelViews;
@@ -109,6 +124,7 @@ private:
 	QWidget* m_channelAreaWidget;
 	QStackedLayout* m_racksLayout;
 	QWidget* m_racksWidget;
+	Mixer* m_mixer;
 
 	void updateMaxChannelSelector();