add ColorGradient and ColorGradientStop

still to do: propertization and serialization
refine ColorGradient

add more constructors, add interpolateColor method
add toImage method
implement interpolateColor for Matze's ColorGradientType
add new files to cmakelists; improve discrete and matze's mode
fix Matze's type
revisit formatting
add ColorGradientProperty.h (not yet tested)
add setStops call
setup tests for ColorGradient and ColorGradientStop; add Libzeug dependency; small changes
improve and fix ColorGradient class based on tests; add more tests
add ColorGradientProperty test; add == and != operators; fix ColorGradientProperty
add more tests
increase precision of linear interpolation; update interfaces; add bits() and bitsF(); update tests
disable warnings as errors for now
move gradient type into color gradient class and use string for type serialization
update formatting
add ColorGradientProperty to CMakeLists.txt
Merge branch 'master' into color_gradient

Conflicts:
	source/glwhite/CMakeLists.txt
	source/tests/glwhite-test/CMakeLists.txt
fix copyright sign