Make undo history a delegate

[fluffy-critter]

Per the work on #100 it's become clear that Qt really wants its object ownership hierarchy to be its own thing, rather than maintaining parallel references in both Python and C++-land.

AlbumEditor's undo history is currently done as a parallel reference in that way, and needs to be turned into a delegate, which should be pretty straightforward, at least.

This would likely be done by creating an AlbumDataDelegate class which contains AlbumEditor's data, undo_history, and redo_history members, and the record_undo method.