[Platform] Move Import and Export to submenus

For nasa/mct#235 in support of nasa/MCT-Plugins#111

* Move current Export options (export to image and export to xml) to a common submenu. 
* Move current Import options (import xml) to an Import submenu for parity & future extension.

     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: Self-review by pull request
     Describe potential impact on other areas of the code: Menus
     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