Make Gradients with Fg/Bg colors editable by users

1. Conflicts:
2. libs/pigment/resources/KoStopGradient.cpp
3. libs/ui/kis_autogradient.cc
4. libs/ui/kis_autogradient.h
5. libs/ui/kis_canvas_resource_provider.cpp
6. libs/ui/kis_control_frame.cpp
7. libs/ui/kis_stopgradient_editor.cpp
8. libs/ui/kis_stopgradient_editor.h
9. libs/ui/widgets/kis_gradient_chooser.cc
10. libs/ui/widgets/kis_gradient_chooser.h