App Settings Dialog should be a property of MainWindowWithProgressDialogs

[lauramurgatroyd]

https://github.com/paskino/qt-elements/blob/bc3c7454672dcfa64d1540af14dbcf918815dbaa/eqt/ui/MainWindowWithProgressDialogs.py#L114-L121

When the dialog is created, it is not saved as a member of self so can't be accessed by a child class to add further elements unless the child class overrides this create method.