Math: simplified Vector*::*Axis() and Vector*::*Scale() functions.
Color: internal helper function renaming.

Will be used also elsewhere than for alpha only.
Various minor cleanup.
Properly test all Color constructors.
Math: test that constructing vector from parts can be done implicitly.
Updated Color documentation with (de)normalization info.
Math: be more explicit in what we want to construct.
Added Color*::{red,green.blue,cyan,magenta,yellow}() convenience functions.
SceneGraph: added TODO.