Add a node wrapper item.
Finish documentation of declarative api.
Fix item deletion bug in sequential layout element.
Add item argument to space between element getter function.
Modify node wrapper to serve as stretchable background item.
Re-implementation of VLoopStatement using declarative API

- remove header, conditionBackground, initStepBackground, updateStepBackground properties
- don't have to override determineChildren method anymore
- don't have to implement destructor anymore
- need to adjust loop handler (HLoop.cpp) because header property was removed
- move shape in the default style out of the layout property
Re-implementation of VIfStatement using declarative API

- remove header, conditionBackground, content properties
- don't have to override determineChildren anymore
- don't have to implement destructor anymore
- move shape in the default style out of the layout property
- need to include specific item types of icon and condition in HIfStatement.cpp
- updateGeometry remains unchanged, since the decision whether to display the branches
  horizontally or vertically needs to know the size of the items
Re-implementation of VMethod using declarative API

- remove header and content properties
- don't have to override determineChildren anymore
- don't have to implement destructor anymore
- move shape in the default style out of the layout property
- need to adjust method handler (HMethod.cpp) because content property was removed
Re-implementation of VClass using declarative API

- remove header, content, fieldContainer, publicFieldArea, privateFieldArea, protectedFieldArea, and defaultFieldArea properties
- manage the different field types as node lists and display them using a SequentialLayoutElement
- don't need to implement destructor anymore
- considerably shorter constructor and determineChildren, but they are still needed to implement the special behaviour of the body and the field background item
- move shape in the default style out of the layout property
Re-implementation of VList using declarative API

- don't need to override determineChildren anymore
- need to modify list handler (HList.cpp), because layout property was removed
- need to modify StringOffsetProvider.cpp, because layout property was removed
- move shape in the certain styles out of the layout property (VArrayInitializer, VLambdaExpression, VMethodCallExpression)