Clickable Yield Icons in toppanel

**Issue by [ProstoSanja](https://github.com/ProstoSanja)**
_Thursday Aug 10, 2017 at 13:18 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/pull/611_

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Decided to start contributing to such a great mod, and to get a little better understanding of how it works in civ6, started with easy feature implementation, which is desribed here: #23

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_**[ProstoSanja](https://github.com/ProstoSanja)** included the following code: https://github.com/CQUI-Org/cqui/pull/611/commits_

Start by reading issue #23 and the commits from the referenced pull request #611; the issue provides no file or test path. Confirm whether the clickable yield icons are already covered by that work, then verify that the toppanel yield icons respond to clicks as intended.

Written by the indexing model from the issue text.