Remove SampleLoader (#8186)

Removes `SampleLoader`. File dialog functions were moved into `FileDialog`. Creation functions were moved into `SampleBuffer`.

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Co-authored-by: Dalton Messmer <messmer.dalton@gmail.com>

include/FileDialog.h

@@ -50,6 +50,8 @@ public:
 									const QString &directory = QString(),
 									const QString &filter = QString(),
 									QString *selectedFilter = 0);
+	static QString openAudioFile(const QString& previousFile = "");
+	static QString openWaveformFile(const QString& previousFile = "");
 	void clearSelection();
 };