Impl. EditText widget for Matrix

- New Widget which contains an edittext for each matrix element
- self-contained parsing on changes for error highlighting
- moved onTreeWidgetItemDoubleClicked and onTreeWidgetItemChanged to AronTreeWidgetController.h, these GUI actions only concern the Tree widget details, not however the full scope in SkillManagerMonitorWidgetController.h
- in AronTreeWidgetCreator.h: createSimpleTreeViewWidget() now called insertNewTreeViewWidget(). It also directly adds the created widget to the parent, has control flow reasons to add it directly.
 (otherwise we cannot directly overlay custom widgets inside the AronTreeWidgetCreatorVisitor)
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