Duplicate fields in enumeration

[Cécile Camillieri]

When adding a field in an enumeration, it prevents the user to add a field with a duplicate name. However when renaming a field, it doesn't, so we can end up with the same field multiple times in the enum. It should do the same checks when renaming a field as when we add one.

[Cécile Camillieri]

References #283: prevents editing an enum literal with a name that already exists

→ <<cset a845c177968a>>

[Cécile Camillieri]

References #283: fixes literals not being trimed during checks when adding several literals

→ <<cset eefd24072290>>

[Cécile Camillieri]

Merged in issues/283-duplicate-literals-in-enum (pull request #23)

References #283: prevents editing an enum literal with a name that already exists

→ <<cset c7323c9a1997>>