"Derived" resources are the resources that do not exist temselves.
Instead they are contained in some other resources and are updated
synchronously with the parent resources as well.

E.g. we store opacity and composite op and opacity in the current
paintop preset, which is also a resource. So composite op and opacity
are "derived" resources.

The main goal of this refactoring is to make our resources comply with
a general Model-View-Controller architecture:

Model: KisPaintOpPreset. It stores opacity, composite op, eraser mode
       and other "global" properties.

Controller: KoCanvasResourceManager. It controls access to the resources
            and emits notification signals when they are changed.

View: KisPaintOpBox and other classes that show the resources on screen

Please take into account that according to the MVC design all the access
to the model resources should be performed through the controller.