Better unit promotion indication

**Issue by [chaorace](https://github.com/chaorace)**
_Thursday Jan 05, 2017 at 19:21 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/219_

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The unit promotion UI is a little lackluster right now and easy to miss. I'd like to add some kind of animation to the unit panel that comes up whenever a unit is selected that's hard to miss. Maybe something like having the XP bar cycle through rainbow colors when ready to promote?

Start by locating the unit promotion UI and the XP bar implementation; the issue does not name files, tests, or an entry point. Define an animation that clearly signals promotion readiness, then verify it appears when a selected unit can be promoted without disrupting the unit panel.

Written by the indexing model from the issue text.