adding we_undo to the ContentDB (or merge it with WorldEdit)

[Lemente]

I believe we_undo should be present on the ContentDB, since it is such an important feature.

Even better would be to merge it directly with Worldedit, if that's possible.

Thanks!

[HybridDog]

I added it to the ContentDB (Ready for Review) now.
Merging it into WorldEdit would be fine for me, but I doubt it would be accepted because the code can be simplified there and Pull Requests don't seem to get much attention, see for example Uberi/Minetest-WorldEdit#92.