pdImages can now read/write their data to XML format

Necessary for forthcoming PDI ("PhotoDemon Image") file format support.
pdLayers can now read/write their data to XML format

Necessary for forthcoming PDI ("PhotoDemon Image") file format support.

Note that these functions will only write out the layer headers to XML
format.  The actual image data for a layer is handled separately, as it
would obviously be ridiculous to convert that much binary data to XML!
Update XML class with functions for operating directly on strings

...instead of requiring source/destination files.
Update Autosave engine so PD will compile

Autosaving is going to be woefully broken until Layers are finished -
sorry!
pdPackage files can now be assembled and written to file

This commit represents one of my favorite pastimes: writing enormous
chunks of code before I have any way to actually test them!  (/s)

Actually, assembling pdPackages is made much easier by the use of an
excellent buffering class from vbforums user dilettante.  His class
provides something VB6 inexplicably lacks: a stream interface for
interacting with a rolling collection of binary data.  His class is very
well-written and commented, and it has made this project much more
enjoyable than it would have been otherwise.  Many thanks, dilettante!