Add 'AvailableOptions' query class for UI Tester

In some cases it may be useful to be able to query what the available options in a enum editor, or checklist editor are for example.
This issue was motivated by traitsui examples: https://github.com/enthought/traitsui/blob/master/traitsui/examples/demo/Dynamic_Forms/dynamic_selector.py and https://github.com/enthought/traitsui/blob/master/traitsui/examples/demo/Advanced/Dynamic_EnumEditor_demo.py
where the available options to be selected are dynamic and depend on the gui state. When writing a test, such a `query.AvailableOptions` class would be useful to verify if the set of available choices is correctly updating.