Add visual indicator to main city state menu indicating quest

**Issue by [brianschamel](https://github.com/brianschamel)**
_Wednesday Mar 08, 2017 at 20:20 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/383_

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To be clear, the screen I'm talking about is the one with the list of city states, not the summary for a single city state.

If it were simply text, it could be shortened ("$CS wants you to train a spearman" -> "Train Spearman" or simply "Spearman" and placed next to the city's name (in between it and the number of envoys)

Alternately, the "CS has a quest" icon could be replaced by a unique icon for each quest, which can hopefully be pulled from the games files as they exist.

Train A Unit quests could be the unit's icon.
Build A District quests could be the district's icon.
Great Person quests could be the icon of the type of Great Person.
Civic/Tech Boost quests could be the icon of the civic/tech.
Send a Trade Route quests could be the trader icon.

Of course, hovering over the icon would still show the text it does now.

Start by locating the main city-state list UI and the existing city-state quest indicator, rather than the single-city-state summary. Review how the current hover text is supplied and how the listed quest types map to available icons; done means the list visibly identifies the quest while retaining the existing explanatory hover text.

Written by the indexing model from the issue text.