Label cities currently occupied another civilization

**Issue by [JHCD](https://github.com/JHCD)**
_Sunday May 21, 2017 at 07:21 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/538_

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I have a feature request for a new label/symbol on the city banner.

Actually it is a stupid work to figure out wich city is occupied by another civilisation.
You have to go through all gossip again or look up if a cityname belongs to the actual owner.
A label would be nice for using a relief Casus Belli or increase diplomatic relations to the original owner.

Start by locating the Lua code that builds the city banner and identify how the current city owner is exposed to the UI. Trace the existing city and civilization data used there, then add the requested occupied-city label or symbol. Done means occupied cities are visibly distinguishable on their banners without requiring gossip or ownership lookup.

Written by the indexing model from the issue text.