Auto-completion only proposes labels for which there exist rules

Use case identified in ISSA2:

 In the auto-completion of labels entered by the user, the user should be allowed to select only labels for which there exists at lease one annotation rule. Two options:
- either do not show the labels for which there is no association rule, or
- or show all the labels but those for which there is no association rule should be grey and no selectable.