[Async DB] Modularize Zotero.DataObject.save()

At this point, I just mostly want to check if this is an OK direction.

What this allows us to do is, after extending, say, a `Zotero.Item` class, to override the `_saveData` method in order to write additional data before the DB transaction is committed. It also removes a bunch of boilerplate code from `DataObject` child classes (there's probably more that can be removed)
If we're going to use ```Zotero.Utilities.Internal.defineProperty``` everywhere, maybe it should either go or be aliases on ```Zotero``` itself, like ```Zotero.debug```. ```Zotero.defineProperty``` is a lot more palatable for something so common.