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Support loading and storing image metadata

See #59 for detailed tracking of this issue.

With this commit, any compatible image metadata is now correctly loaded
and stored at image load-time.  (By "any" I mean - any metadata
supported by FreeImage for any filetype.)

Metadata is stored in a new class called pdMetadata.  Each pdImage
object has one public pdMetadata object named "imgMetadata".

For testing purposes, the Effects -> Test menu (only visible in the IDE)
can be used to enumerate all loaded metadata tags and display their
contents via message box.

NOTE: as of now, no attempt is made to write these metadata tags back
out to file at save time.  That will come later.