Renaming the <layer-> element to <map-layer>

[prushforth]

Have received some comments over the years that the <layer-> element is weird, so we're going to rename it to <map-layer>. There are PRs open here, here, here, here, here and here. There are other repos that could be updated for that change and many other changes as well, but since they are languishing at this time, I propose to not update them until they are up for renewal. There may be web resources like blogs and so on that could do with an update as well. Please add any suggestions or comments below. We will likely merge those changes when ready, followed quickly by release 0.15.0.