libpaintop: shared functionality for brush engines

libpaintop is meant to share common classes used by Krita brush engines, suchs as certain configuration widgets.

Options

A paintop can make use of the predefined options provided here. An option provides a graphical view for editing that option (as a page in an option popup) and possible also the code necessary for applying the effects of that option to the paint device as part of painting.

The following classes are important:

• KisPaintOp: this is the engine for a certain kind of brush. It will be created in non-gui threads and keeps its state in a KisPaintOpSettings object.
• KisPaintOpSettings: this is a KisPropertiesConfiguration subclass that stores the settings for all the options that make up a paintop.
• Classes derived from KisPaintOpOption. These classes create a configuration widget. That means that you cannot construct any those objects in a KisPaintOp. KisPaintOps are created in non-gui threads.
• Classes derived from KisCurveOption. KisCurveOption classes have a generic GUI widget, KisCurveOptionWidget. So, in contrast to KisPaintOpOption classes, KisCurveOption instances can and will be created in the constructor of KisPaintOp paintops. This class can manage to read and write its settings directly.
• Classes derived from KisPaintopPropertiesBase. These classes are non-gui, and can read and write KisPropertiesConfiguration objects created by KisPaintOpOption.