If a label title contains `::` then everything until last `::`
(greedy matching) is considered as a label 'key' and labels with
the same key are mutually exclusive:
* if multiple labels with the same key are added, only last selected
is added, also any existing labels with the same key are removed
* when removing an exclusive key, and multiple exclusive keys are
assigned (from before we introduced exclusive keys), all other
exclusive keys are kept