[Quit] Add "Quit" menu item

Implements  nasa/mct#114 - Added "Quit MCT" menu item which closes all open MCT windows and exits the applications.

Also moved some dialog calls to WindowManager to decrease number of explicit Swing dependencies of MCT actions, small steps toward nasa/mct#154 and nasa/mct#155

Refactored existing shutdown code (invoked when last window is closed) to match "Quit MCT".

Also moved "System.exit" out of GUI code - this becomes a responsibility of the platform bundle's lifecycle. 

Moved "commit or abort" dialog invocation so that it is invoked when closing multiple windows at once (not just when closing the window using the native windowing system's close button)

 Wrote or re-wrote unit tests (Y/N): Y 
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Code review by pull request
 Describe potential impact on other areas of the code: Closing windows, quit
 Describe any test prerequisites for this fix (e.g., restart the database): N/A
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N