Add guard to safely reset bools

Add the new local class `BoolGuard` which uses RAII to safely set a
boolean to a value and later reset it to the old value.

The guard is needed to ensure that the boolean `m_presetMode` is safely
set to `true` in `savePreset` and `loadPreset` and reset even in case of
exceptions.

For this to work `m_presetMode` had to be changed to protected
visibility. On this other hand this enables the removal of
`setPresetMode`.

Add comments to the two places where previously the mode was reset so
that people do not wonder why nothing is reset and potentially put the
code back in there.

include/Track.h

@@ -206,10 +206,9 @@ public slots:
 
 protected:
 	bool isPresetMode() const { return m_presetMode; }
-	void setPresetMode(bool presetMode = true)
-	{
-		m_presetMode = presetMode;
-	}
+
+protected:
+	bool m_presetMode = false;
 
 private:
 	TrackContainer* m_trackContainer;
@@ -224,8 +223,6 @@ public slots:
 	BoolModel m_soloModel;
 	bool m_mutedBeforeSolo;
 
-	bool m_presetMode = false;
-
 	clipVector m_clips;
 
 	QMutex m_processingLock;

src/core/Track.cpp

@@ -218,7 +218,7 @@ void Track::saveSettings( QDomDocument & doc, QDomElement & element )
 
 	if (isPresetMode())
 	{
-		setPresetMode(false);
+		// No need to unset preset mode here as this will done by the guard in InstrumentTrack::savePreset
 		return;
 	}
 
@@ -279,7 +279,7 @@ void Track::loadSettings( const QDomElement & element )
 			node = node.nextSibling();
 		}
 
-		setPresetMode(false);
+		// No need to unset preset mode here as this will done by the guard in InstrumentTrack::loadPreset
 
 		return;
 	}

src/tracks/InstrumentTrack.cpp

@@ -994,15 +994,45 @@ void InstrumentTrack::loadTrackSpecificSettings( const QDomElement & thisElement
 	unlock();
 }
 
+/**
+ * @brief RAII "guard" used to safely change and reset booleans even during exceptions
+ * 
+ * Needed to safely reset m_presetMode in savePreset and loadPreset. For this reason
+ * only defined locally because at least the pattern used by these methods should not
+ * spread outside.
+ */
+class BoolGuard
+{
+public:
+	BoolGuard(bool& member, bool temporaryValue) :
+	m_member(member),
+	m_oldValue(member)
+	{
+		m_member = temporaryValue;
+	}
+
+	~BoolGuard()
+	{
+		m_member = m_oldValue;
+	}
+
+	BoolGuard(const BoolGuard&) = delete;
+    BoolGuard& operator=(const BoolGuard&) = delete;
+
+private:
+	bool & m_member;
+	bool const m_oldValue;
+};
+
 void InstrumentTrack::savePreset(QDomDocument & doc, QDomElement & element)
 {
-	setPresetMode();
+	BoolGuard guard(m_presetMode, true);
 	saveSettings(doc, element);
 }
 
 void InstrumentTrack::loadPreset(const QDomElement & element)
 {
-	setPresetMode();
+	BoolGuard guard(m_presetMode, true);
 	loadSettings(element);
 }