PulpLabels: warn when overwriting labels

[himdel]

Adding a foo: bar label and then a foo label causes the first label to be overwritten, and only foo to remain.
(Similarly in the opposite order, only foo: bar remains - the latter option always wins.)

This happens both when editing in one go, and when adding these sequentially.

(I assume that in itself is not a bug, and it's just caused by labels being a hash.)

But we should at least indicate that something like that will happen, for example by making all but the last foo* greyer, or redder, with a title/(?) saying it will get removed. Or something :)