Midi: Remove some duplicated code

Introduced new private read/write_midi_settings() and used them
in MidiSettingsView and SoftSynth.

headers/private/midi/MidiSettings.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2015 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ */
+
+#ifndef MIDI_SETTINGS_PRIVATE_H_
+#define MIDI_SETTINGS_PRIVATE_H_
+
+#include <StorageDefs.h>
+#include <SupportDefs.h>
+
+namespace BPrivate {
+
+struct midi_settings {
+	char soundfont_file[B_FILE_NAME_LENGTH];
+};
+
+status_t read_midi_settings(struct midi_settings* settings);
+status_t write_midi_settings(struct midi_settings settings);
+
+};
+
+
+#endif /* MIDI_SETTINGS_PRIVATE_H_ */