gegl-color: Add documentation about format of accepted strings
tests: Add unit tests for GeglColor parsing

Currently failing as named and RGB hex does not interpret values as gamma-corrected.
gegl-color: Take gamma-corrected colors when on #rrggbb format and for named colors

This changes the semantics to be line with the expected for CSS/HTML values.
Because the format can only have 256 values per channel, gamma-corrected
is also the only thing which gives sufficient perceptual precision.
Makes previously added unit tests pass.

Warn: changes output of operations using this type of format strings!