UI for increment #1

The listbox for showing the stack does not show it bottom-up, but that
should be ok for now.
RPNCalculator for increment #1
Refactored RPNCalc to return result through event instead of return values
Wired up UI and RPNCalculator for increment #1

Application running in a simple way
UI for increment #2
RPNCalculator for increment #2
Updated integration for increment #2

The application now supports addition
UI for increment #3