Adds leaf cross section

[XylotrupesGideon]

Adds the leaf cross section from #79

[duerrsimon]

Might be more appropriate in the plant category? What do you think?

[XylotrupesGideon]

Hmm I think it is a classical case of double category.
I thought the plant section would be used for icons of full plants in the same way as the animal section has full animals and then the different tissues of these organisms go to the tissue category. I think the animal category would get very crowded if we add icons of all different tissues and organs over time.
That's why I decided on the tissue category here...
Maybe an explicit description of what goes into which category would be useful for contributers

[duerrsimon]

Also here the comment, that multi categories are coming!

For now I would still put it in plants since the two other cross sections are also in plants. I agree with your reasoning but best to be consistent for now.