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.