[Quit] Add Quit menu item, nasa/mct#114
[Quit] Add shutdown warning to Quit menu item
[Quit] Move shutdown dialog text to resource bundle
[Quit] Move System.exit out of quit invocations
[Quit] Quit action is valid even without target housing
[Quit] Move dialog to WindowManager to reduce coupling of quit action to GUI framework
[Quit] Let Swing supply icon for known message types
[Quit] Use WindowManager's dialog when shutting down by closing last window
[Quit] Add unit tests for QuitAction
Merge pull request #157 from VWoeltjen/mct114

[Quit] Add "Quit" menu item