Enable dragging notifications to rearrange

**Issue by [chaorace](https://github.com/chaorace)**
_Monday Jan 09, 2017 at 08:18 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/230_

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The production queue already has this, so it shouldn't be too difficult to replicate that functionality here. See ProductionPanel.lua for the relevant logic

[More info](https://www.reddit.com/r/civ/comments/5mqlkl/long_feedback_on_civ6_from_a_longtime_series_fan/)

Start by reading ProductionPanel.lua and the existing production-queue drag-and-rearrange logic it contains, then locate the notification UI that should receive the same behavior. Done means notifications can be dragged to change their order, matching the production queue's existing functionality.

Written by the indexing model from the issue text.