Fixed a memory issue in PLNavigationPopUpButton and PLPythonIntrospectionController.

In PLNavigationPopUpButton, no longer release the dataSource and delegate as they are assigned properties. In PLNavigationIntrospectionController, added two missing Py_XDECREF calls to attributes retrieved from a Python object.
Fixed an issue with the navigation popup button where the title would disappear.

Refactored PLNavigationPopUpButton to manually set its cell's menu item, rather than use the selected item. This allows better control over setting the No Navigation or No Selection title, which are now menu items stored as instance variables. This commit also includes minor cleanup within the class, including setting the target/action of the button rather than each menu item individually and converting the sortNavigationRanges: method to a function.