Interface to allow sharing of graphics settings between controls.

The interface provides functions to set the graphics context of a control before drawing. If a control has its iContext member set, the UI Control Framework calls functions defined by this interface when a control is about to be drawn. Developers must implement PrepareContext(), which is called by the framework, to initialise the control's window with the required graphics settings.

To use control contexts, a control should inherit from an MCoeControlContext-derived class. To share the context between controls, this control should then be set as the context for all controls that wish to share it. This is done by setting the iContext member of each of the controls, using CCoeControl::SetControlContext() and CCoeControl::CopyControlContextFrom().