Show a warning when a branch length option (e.g. ignore lengths; make ultrametric) doesn't actually change the tree

[fedarko]

Discussed with @gwarmstrong in #444 at #444 (comment). Basically, if the user has a tree that is already ultrametric, then clicking the "make ultrametric" option should show a warning -- and if the tree's nodes all already have the same length, then clicking "ignore branch lengths" should also show a warning (I thiiink that even if the uniform length is 2 or something the drawing will still look identical to the uniform length of 1 used under the hood in the layout computations, due to how the camera/etc are scaled).

I think the most likely cause of users running into this situation in practice will be Qemistree users coming in with ultrametric trees and wondering why the Make ultrametric option doesn't change anything. Fortunately, the UI for these options (esp. with the improvements coming soon in #444) should make this situation pretty obvious, so I don't think this is a very high-priority issue.