Fixes 0010592: mc_enum functions doesn't take customization into account

The mc_enum_xxx functions have been updated to return the customised
keys, while the names remain the translated ones.

mc_enum_get has not been updated, as it returns a raw enum string, not
an ObjectRefArray.

Unfortunately no new tests can be added since we don't control the
customisation settings on the mantis installation on a per-test
basis.

• split functionality from mc_issue_get
• explode_bla_bla now takes a enumeration name
• localized labels are properly used