Convert the Undo/Redo engine from Passive to Active strategy

Details available in the pdUndo class header.  In a nutshell, PD will
now create Undo/Redo data *immediately following any action that
modifies an image."  This means that the first Undo/Redo file is created
as soon as an image is loaded. Previously, PD would only create
Undo/Redo data *immediately prior to* any action of note, which meant
that if all you did was load an image then close it, no Undo data was
ever created.

There are trade-offs to both methods, but an Active strategy is a much
better fit for my forthcoming work on "save the bare minimum Undo/Redo
data necessary to reconstruct image state".

Selections may or may not work with the new strategy; re-integrating
them will happen after some other Undo/Redo engine changes are
implemented.