Update master image load function in light of recent design decisions

- I'm now committed 100% to 32bpp by default for all layers.  While this
introduces some overhead, it is the standard behavior in other software
for a reason - anything that involves chopping out bits of the layer
will automatically behave appropriately under this model, and we can
skip the messy business of separate code paths for 24 or 32bpp data.

- Use better branching code for ignoring processing steps PDI files
don't need (like ICC profile handling, which will have already been done
when the PDI file was created)
New "UNDO_EVERYTHING" Undo type

This is required for the Undo/Redo diff engine; the first undo file
created must cover *everything we know about the image*, so at present,
that means a full copy of the pdImage stack, and a default Selection
file.

This initial entry will be used as a baseline for all subsequent
Undo/Redo diffs, and hopefully we can easily use it to reconstitute the
image at any point in time (for an Undo History, among other things).
First successful diff-based Undo implementation

Awwww yeah, this is exciting.  Selection Undo operations have now been
successfully implemented using diffs (instead of a full pdImage stack
copy).  This can be verified by assigning PD its own temp folder, then
examining the contents of the temp folder as you run PD and intermix
selection and image-based actions.  Unlike before, where every action
created a full copy of the image and the selection data, there will now
be a bunch of 1kb selection files intermixed with much larger image
files.  This makes all selection-related UI actions way more responsive,
and saves the user a great deal of hard drive space!

With the underlying Undo/Redo diff engine now operational, the next task
is to implement all the different diff types (image, image header,
layer, layer header, etc), and in some cases, to write custom functions
for things like "save just the layer header to file instead of the full
header + bitmap data".  This work will be tedious, but I think the worst
parts are over.

Once all diff types are implemented, I can finally add Undo support for
things like moving layers on-canvas.