When sorting labels by user selection order, any missing labels
had index 0. If only single selected label was returned by UI,
then this label had index 0 too so it was not chosen as the "last
selected" label. To fix this, any missing labels have index -1.

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