Add peak indicators (#7295)

Add peak indicators to the mixer strips. They show the maximum peak value
that was observed and can be reset by clicking on them.

## Implementation details
The implementation works via a signal/slot mechanism. The `Fader` class
has a new signal `peakChanged` which reports peak values as
amplifications. A new class `PeakIndicator` is added which has a slot
`updatePeak` which is connected to the new signal in `Fader`.

The `PeakIndicator` inherits from `QLabel` and mainly deals with updating
the label text from the current peak value.

Add a `PeakIndicator` instance to `MixerChannelView`. Add a `reset`
method to `MixerChannelView` so that the mixer channel can be reset on
the loading of new projects, etc. The current implementation resets the
peak indicator back to -inf dbFS. The `reset` method is called in
`MixerView::clear`.

Remove the clamping in `Fader::setPeak` so that all peaks are reported.
Emit the new signal if the peak changes.

include/Fader.h

@@ -105,6 +105,9 @@ public:
 		setUnit(txt_after);
 	}
 
+signals:
+	void peakChanged(float peak);
+
 private:
 	void contextMenuEvent(QContextMenuEvent* me) override;
 	void mousePressEvent(QMouseEvent* ev) override;

include/MixerChannelView.h

@@ -45,6 +45,8 @@ namespace lmms
 
 namespace lmms::gui
 {
+    class PeakIndicator;
+
     constexpr int MIXER_CHANNEL_INNER_BORDER_SIZE = 3;
     constexpr int MIXER_CHANNEL_OUTER_BORDER_SIZE = 1;
 
@@ -90,6 +92,8 @@ namespace lmms::gui
         QColor strokeInnerInactive() const;
         void setStrokeInnerInactive(const QColor& c);
 
+        void reset();
+
     public slots:
         void renameChannel();
         void resetColor();
@@ -119,6 +123,7 @@ namespace lmms::gui
         QLabel* m_receiveArrow;
         PixmapButton* m_muteButton;
         PixmapButton* m_soloButton;
+        PeakIndicator* m_peakIndicator = nullptr;
         Fader* m_fader;
         EffectRackView* m_effectRackView;
         MixerView* m_mixerView;

include/PeakIndicator.h

@@ -0,0 +1,60 @@
+/*
+ * PeakIndicator.h - Peak indicator widget
+ *
+ * Copyright (c) 2024- Michael Gregorius
+ * 
+ * This file is part of LMMS - https://lmms.io
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program (see COPYING); if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301 USA.
+ *
+ */
+
+
+#ifndef LMMS_GUI_PEAKINDICATOR_H
+#define LMMS_GUI_PEAKINDICATOR_H
+
+#include "lmms_export.h"
+
+#include <QLabel>
+
+
+namespace lmms::gui
+{
+
+class LMMS_EXPORT PeakIndicator : public QLabel
+{
+	Q_OBJECT
+public:
+	PeakIndicator(QWidget* parent);
+
+	void resetPeakToMinusInf();
+
+public slots:
+	void updatePeak(float peak);
+
+protected:
+	void mousePressEvent(QMouseEvent* e) override;
+
+private:
+	void updatePeakDisplay();
+
+private:
+	float m_peak;
+} ;
+
+} // namespace lmms::gui
+
+#endif // LMMS_GUI_PEAKINDICATOR_H