Add ability to have animated color changes on a per-tile basis

[SquidPony]

This would mean setting a start and end color (or perhaps keyframe colors) and a requested timing value. In Swing the timing value would only be very approximate, but for single events this could be useful.

[tommyettinger]

This should be possible currently with TextCellFactory.Glyph Actors, which both SquidPanel/SquidLayers and SparseLayers use for animated chars (including chars that move smoothly). I'll add a convenience method to at least one of SquidPanel and/or SparseLayers to promote a char to a Glyph and start a ColorAction on it, since ColorAction is part of LibGDX but isn't especially clear that it's available to users.