Add and display 13 new Canon tags

[kwhopper]

Delved into Exiftool a bit to identify some Canon tags and offer some potential display cleanup for a few existing tags.

New/Revised Canon: TagRecordMode, TagMaxAperture, TagMinAperture, TagFocusContinuous, TagAESetting, TagDisplayAperture, TagZoomSourceWidth, TagZoomTargetWidth, TagSpotMeteringMode, TagPhotoEffect, TagManualFlashOutput, TagColorTone, TagSRAWQuality

A couple of functions were also converted from Exiftool to C# and I tried to attribute those appropriately.

[drewnoakes]

Between now and the time I get to review this, perhaps you could try running it over the sample images?

Specifically, clone this (large) repo, then run MetadataExtractor.Tools.FileProcessor.Program.Main with argument --text "<path>\metadata-extractor-images". Then run a git diff on the repo. You will see some changes in the *\metadata\*.txt files.

The sample images repo contains the output of the Java library. The .NET library currently produces some differences around text encoding and some time zone stuff. But if you run the above with your changes, you'll be able to see if it's introduced any other more significant changes to the output for any of the sample images. It's quite common that changes work on one camera model but introduce breaks on others. I'd have to run these checks before merging this PR, so it'd be smoother if you ran them yourself. The only problem is that the repo is quite large, but you only have to clone it once.

[kwhopper]

Ok - I went through all the images per your instructions. There were no new errors that I could see; most files changed mod dates but had no textual changes. Text and datetimes were different, but that was expected. "Focal Plane" changes tended to show up the most outside Canon-model images.

Otherwise, I couldn't find anything new or that wasn't expected output. If there's something else to check let me know.

[drewnoakes]

Thanks, I've merged this as 444f19c. Nice work on those tricky AF tags. The documentation you added makes the strange logic clear.