Saving the app

This operation is performed by the common.app_tree Tree: for every Node of the tree, an 'object' xml element is generated, with the following attributes: name, class, base (class). Each object contains an element for each Property (generated by the write method of Property) and then an 'object' element for all its sub-widgets and/or sizers. Properties in the sizer_properties dictionary are treated in a different way, as well as the children of a sizer, which are sub-elements of sizeritem objects: see the source code for details.