Added GetFrame(), GetFrames() and GetArtworkImage()

These functions address several issues related to poor handling for
multiple frames of the same type, especially for multiple artwork
frames. Tag Viewer now allows viewing all of the embedded artwork images
using Previous/Next or by selecting the APIC/PIC frame from the ID3v2
Data panel's list.

* GetFrame("APIC") returns the first APIC frame.
* GetFrame(1) returns the second frame in the tag.
* GetFrames("APIC,PIC") returns all artwork frames matching these two
types.

Also removed a large amount legacy code that had been commented out but
kept for temporary reference.
Merge branch 'master' of https://github.com/perrybutler/id3taglibrary