Add support for boxed display of property widgets
Previously, property widgets have always had their components displayed horizontally next to a label with the property name. However, that made widgets consisting of many components (e.g. widgets for vector properties) hard to use because their components were squeezed together. Now AbstractPropertyWidget takes one additional optional argument, displayBoxed. If it's false, the widget is displayed as before. When it's true, the widget is laid out vertically in a QGroupBox. For now only widgets for vector properties set displayBoxed to true.