Changing the value of a Property

When the user selects a widget the property window changes to display the properties of the selected object: this is done by the functions show_properties of edit_windows.EditBase and edit_sizers.SizerBase, which are called inside two event handlers for focus and tree selection events.

When the value of a Property is changed, its setter function is called to update the aspect/layout of the widget the Property belongs to: such function is obtained from a call to the widget's __getitem__ method, which must return a 2-tuple (getter, setter) for the Property