FormDialog and UIFormWidget do not support widgets that inherit from QWidget only

[jcornall]

When trying to add a widget that is a subclass of the QWidget class only, a number of methods in the UIFormWidget class (i.e. getWidgetState(), applyWidgetState()) cannot execute correctly because the required class methods do not exist in the target class. Widget classes such as QLabel, QLineEdit and UISliderWidget are supported, however no such support exists for classes that only inherit from QWidget.

If a class is defined that inherits from QWidget and creates a layout containing multiple widgets, the UIFormWidget methods cannot 'unpack' the methods and attributes of these 'child' widgets. Additional methods would need to be added to the target class, and the UIFormWidget methods would need to be altered to accommodate this.

See also #125, #173, #175