fixed color unit tests; fixed new color interface (explicit is better than implicit)

modules/gfx/pymod/export_color.cc

modules/gfx/src/color.cc

modules/gfx/src/color.hh

modules/gfx/tests/test_color.cc

modules/gui/pymod/__init__.py

modules/gui/pymod/scene/color_select_widget.py

modules/gui/pymod/scene/gradient_editor_widget.py

modules/gui/pymod/scene/immutable_gradient_info_handler.py

modules/gui/pymod/scene/immutable_info_handler.py

modules/gui/pymod/scene/preset_editor_widget.py