Separate class for extended note range upgrade (#6548)

Extract the code that upgrades the extended note range into its own
class. This hides the helper functions that are related to the upgrade
from the other upgrade methods in DataFile.cpp.

The header file is put into the src/core directory as it is not part of
the public interface and therefore should not be included in the
"global" include directory. The whole thing is just an implementation
detail of the upgrade in DataFile.cpp.

src/core/CMakeLists.txt

@@ -77,6 +77,8 @@ set(LMMS_SRCS
 	core/ToolPlugin.cpp
 	core/Track.cpp
 	core/TrackContainer.cpp
+	core/UpgradeExtendedNoteRange.h
+	core/UpgradeExtendedNoteRange.cpp
 	core/Clip.cpp
 	core/ValueBuffer.cpp
 	core/VstSyncController.cpp