[Quit] Move Quit to bottom of its menu, nasa/mct#114

It was noticed that the Quit menu added for #114 didn't match norms for this menu. Specifically:

- "Quit MCT" appeared above menu items injected by the ImportExportProvider plugin
- "Quit MCT" was not separated from the object-related menu items that appeared above it

This pull request addresses these points. 

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 req
 Describe potential impact on other areas of the code: N/A
 Describe any test prerequisites for this fix (e.g., restart the database): Run with ImportExport plugin installed
 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