Color overlay test images (for wooden tower)
overlays: Start work on ColorOverlayComponent

Most of this is based on inventory animation overlays, some complexity
related to drawing order was (temporarily) dropped in favor of getting
the prototype up and running.

Target colors can either be defined as [r,g,b,a] containers or as
strings. If the latter, they are interpreted as attribute template and,
for e.g. `'owner.color'`, we try to use `self.instance.owner.color` as
target color. Note that this attribute must be either a fife.Color or a
horizons.util.Color object to be correctly recognized.

The `255 - color.a` should probably be generalized as something along
the lines of an "invert_alpha" property.
Support for changing multiple colors per overlay, and multiple overlays

Generalized alpha property in the sense of it simply being definable
through YAML now. Also wow, that docstring.
Allow defining alpha value of fife.Color attributes

Uses new property notation of fife.Color which is not merged into master
just yet.