Suggestion to rename trust levels

The current trust levels are nicely customized for DAML’rs, but I was thinking it would be nice to have them to be even more DAML-centric by using party qualifiers†.

The following have been suggested by @Martin_Huschenbett and @bernhard:

• trust level 0: witness
• trust level 1: observer
• trust level 2: controller
• trust level 3: signatory
• trust level 4: operator

† operator is not really a party qualifier but it’s a well established term for whoever’s running a ledger or a participant

Those customized names are amazing @stefanobaghino-da!