* Created Menu class. It's a non-native alternative to Tk's Menu class
which is easier to control/mix with other Tk elements.
* Created Popup class. Easy way to display secondary, floating, featureless
windows. The Menu class utilizes them.
* Created event concept. Anything marked with the event decorator can be
observed in any of the following ways:
1 - By subclasses, as always.
2 - By whatever is assigned to instance.delegate.
3 - By calling bind<name of event>(functionToCall)
* Entry class has the events: onChange, onReturn, onUp,
onDown, onFocus, onFocusLost.
* Menu class has the events: onHighlight, onSelect.