Implement OpenXmlElementFunctionalExtensions

[ThomasBarnekow]

This commit implements the OpenXmlElementFunctionalExtensions class and
related unit tests.

Fixes #634.

[twsouthwick]

This looks great! Can you update the Changelog with this? I'll merge once that's in

[twsouthwick]

I'm glad this method works without adding so many new constructors. We can explore that option, but if this works, that simplifies the addition of 1000s of new public APIs :)

[ThomasBarnekow]

Just out of curiosity, what exactly would be the problem? If we added new constructors, those would all be implemented in the exact same way, making automatic generation easy. Not sure whether we could replace existing constructors without breaking things.

[ThomasBarnekow]

And, yes, I will update the Changelog.

[ThomasBarnekow]

@twsouthwick, regarding the CHANGELOG, to which version will this be added? 2.11.0 (new feature) or 2.10.1?

[twsouthwick]

It'll be 2.11.0 - we follow SemVer and this adds new surface area. Any revision (ie 2.10.x) is done as a hotfix-style (see the tag for 2.9.1 where we cherry-picked a fix for .NET Native)

[twsouthwick]

The issue is just the number of APIs - not necessarily horrible, but it locks it in as we try not to remove APIs for backwards compatibility.