Add class for displaying stacked bars

This is an AbstractItemView-implementation.
Based losely on the AbstractItemView-Qt-Demo-Application.
Implementation of a QAbstracTableModel to manage the space usage data.
Wire the newly added stacked-bar view & space usage model up.
Replace buggy legend stackedbar/legend drawing code by QLinearGradient et al.
Define and use an enum to address the different entries in the SpaceUsageModel.