[Platform] Add Export submenu

Add Export submenu, into which Export as
Image and Export as XML can be moved.
[Platform] Move Export as Image to Submenu
[Platform] Add Import submenu
[Platform] Hide Import submenu by policy

Hide the Import menu when the user cannot
write to the component which would be the
target of the Import action, as discovered
by policy.
[Platform] Support extension from Import/Export submenus

Indicate to platform that the appropriate extension
points are available for the Import and Export
submenus. This allows plug-ins (such as the XML
ImportExportProvider) to inject there.
[Import/Export] Use submenus from Platform

Utilize Import and Export submenus introduced at
the Platform level for XML Import/Export menu
items published by ImportExportProvider.
[Import/Export] Make action names appropriate for submenu

Change names for Import and Export actions (for XML and
Images) such that they appear appropriately and
consistently in the Import and Export submenus
introduced for nasa/mct#235
[Platform] Add tests for Import/Export menus

Add unit tests for Import and Export submenu
extension points in the platform. Enhance
logic of menus to account for cases considered
while writing tests.
[Import Export] Enhance unit tests for Import action

Improve test coverage for Import actions.
Merge pull request #236 from VWoeltjen/mct235

[Platform] Move Import and Export to submenus