Add a way to set graphics in a header/footer

[imirkin]

The full functionality of the VML drawing is not addressed here. However this seems to work for embedding regular images.

PR Details

Adds the ability to set header/footer graphics.

Description

This is a somewhat restricted implementation out of all the options possible. But it's enough to get the basics going, and I doubt anyone out there wants too much more than this offers -- image + width/height for scaling.

If this function is called twice, it won't "update" the existing bits, but rather just leave them around and make a new drawing / media image. I don't think there's any process to "GC" the left-overs, but it's also not completely clear what to do about this. The media object may be shared by multiple drawings / sheets that have the same image.

How Has This Been Tested

This is based on what MS Excel produces. I tried other variations but was mostly unsuccessful. I had some variations which produced a border (without using the formulas bit), and many which crashed Excel -- seems like the logic to processing these is a bit fragile. Not 100% sure what all is needed since there are a lot of permutations, but with everything I have, it seems to work OK.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

I wasn't too sure how to test the change, so I added an example.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.20%. Comparing base (0d5d1c5) to head (e735fdd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2018   +/-   ##
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  Coverage   99.20%   99.20%           
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  Files          32       32           
  Lines       29803    29880   +77     
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+ Hits        29565    29642   +77     
  Misses        158      158           
  Partials       80       80           

Flag	Coverage Δ
unittests	99.20% <100.00%> (+<0.01%)	⬆️

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[xuri]

Thanks for your PR. I updated unit tests and simplify code based on your code. Much appreciated.

[imirkin]

Great. I wasn't 100% sure about the API, and I couldn't come up with a good Get variant. But I think it's still useful without that. I thought about putting it into the SetHeaderFooter call, but since there's no good way to Get it, I decided against it. Happy to iterate on the API if you think it still needs work though.

[xuri]

Thanks for your contribution. I merged this PR. If you'd like to add GetLegacyDrawingHF function, that would be great.