Add confirm removal on mixer channels (#6691)

* Add confirm removal on mixer channels

Add confirm removal popup when the user calls the action "remove channel" on a mixer channel that is in use (receives audio from other channel or track). Set a config variable to keep track if the user don't want to be asked again. Adding a scroll on settings-general tab because there weren't enough space on the area.

* Core Mixer function channel in use

New core Mixer function to check if a given channel is in use (receives audio)

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Co-authored-by: Hyunjin Song <tteu.ingog@gmail.com>
Co-authored-by: saker <sakertooth@Gmail.com>

include/Mixer.h

@@ -202,6 +202,10 @@ public:
 	// rename channels when moving etc. if they still have their original name
 	void validateChannelName( int index, int oldIndex );
 
+	// check if the index channel receives audio from any other channel
+	// or from any instrument or sample track
+	bool isChannelInUse(int index);
+
 	void toggledSolo();
 	void activateSolo();
 	void deactivateSolo();

include/MixerView.h

@@ -95,6 +95,7 @@ public:
 
 	// notify the view that a mixer channel was deleted
 	void deleteChannel(int index);
+	bool confirmRemoval(int index);
 
 	// delete all unused channels
 	void deleteUnusedChannels();

include/SetupDialog.h

@@ -82,6 +82,7 @@ private slots:
 	void toggleLetPreviewsFinish(bool enabled);
 	void toggleSoloLegacyBehavior(bool enabled);
 	void toggleTrackDeletionWarning(bool enabled);
+	void toggleMixerChannelDeletionWarning(bool enabled);
 	void toggleMMPZ(bool enabled);
 	void toggleDisableBackup(bool enabled);
 	void toggleOpenLastProject(bool enabled);
@@ -141,6 +142,7 @@ private:
 	bool m_letPreviewsFinish;
 	bool m_soloLegacyBehavior;
 	bool m_trackDeletionWarning;
+	bool m_mixerChannelDeletionWarning;
 	bool m_MMPZ;
 	bool m_disableBackup;
 	bool m_openLastProject;