DESIGN

1. StepCore design is described in Doxygen-generated documentation
which is available on-line at
http://stepcore.sourceforge.net/docs/design_intro.html

2. Step design.

The central object of Step is WorldModel. It contains World object and
provides acts as a proxy for its main functions (add, get and remove items,
change solver, do integration, load and save XML files). It also provides
QAbstractItemModel interface (with all its useful signals) and is directly
used as a model for WorldBrowser. It also provides global selection handling.

Second important object is WorldScene. This one together with *GraphicsItem
classes controls all scene drawing and editing with mouse. Together with 
*Creator classes it controls item creation with the mouse.

WorldFactory class extends StepCore::Factory and provides various item-
specific static functions such as newItem(), newItemCreator(),
newGraphicsItem() and functions to convert StepCore types (currently only
StepCore::Vector2d) to and from QString.

PropertiesBrowser class allows user to browse and edit properties of selected
object. Currently properties are stored using Q_PROPERTY, but it may change
in the future. At that time custom widget for properties of any Item can be
created to allow more convenient presenting and editing object properties.