* Song: add isSavingProject.
* VersionedSaveDialog: Add support for a custom option dialog.
* AutomatableModel: Support the discardMIDIConnections save option.
* InstrumentTrack: Support the discardMIDIConnections save option.
* SaveOptions: Show the save option dialog on "save as".
A file save dialog (inherits FileDialog) that provides buttons to
increment or decrement a version which is appended to the file name.
(e.g. "MyProject-01.mmpz")