Remove old thumbnail-specific EXIF tags when preserving metadata

We embed thumbnails in a JPEG's JFIF header, not the EXIF area, so we
need to make sure to clear out any old EXIF thumbnail descriptors while
we're at it.
Updates to Acknowledgments; so many helpful people! :)
Do not sync new Autosave user folder
New LoadRawImageBuffer function in the master image load routine

Very helpful for the new Autosave function, and probably also good to
have on hand for future expansion.
Create an Autosaves folder if one is not present at load-time
pdImage entries can now write critical data to/from XML

Useful for the new Autosave engine, among other things
Core Autosave framework now in place

After an unsafe shutdown, the next time PD is run, a list of Autosave
entries will be compiled.  These entries are then verified for
integrity, then presented to the user for potential restoration.

Images that were loaded but not modified will not be presented, as they
will not have any Undo/Redo data available.

Images that were loaded but were *not* marked as "already on disk", e.g.
images pasted from the clipboard or imported from a scanner, now have
special autosave data generated when they are first loaded, as a
failsafe.

Throughout the course of normal program operation, pdImages will write
XML-based "autosave" entries to a new Autosave folder (in the user's
/Data subfolder).  These XML entries contain extra information about the
image that might be helpful when reconstructing the original session.  I
don't make full use of that data at present, but theoretically I could
in the future.

There is still some work to be done; specifically, images that user
marks for restoration must actually be loaded!  This is coming soon.
Fix unload behavior of custom PD dialogs
Finish work on Autosave dialog UI

The Autosave dialog will now be displayed to the user whenever Autosave
data is found.  The user can select which images to restore and which to
ignore, and those images will be loaded into PD for further editing.

Still TODO: after loading, restore each image's original settings from
the Autosave XML files (e.g. location on disk, file type, etc), as those
values will erroneously correspond to the temp autosave files, not the
original copies of the files.