Hygiene: Rename _save to onSaveBegin and _prepareForSave to onStageChanges

Since these are events triggered by clicking save/continue and are overriden
they should be public or protected not private.

To be consistent with other names let's prefix them with `on` and
add common lines of code to the parent postRender method

Change-Id: Ic04df0411bfee67b89039e45daae2f95fdc4084d