[Save All] Move getAllModifiedObjects to capability

Moves behavior associated with getAllModifiedObjects to a capability.

Benefits:
* Reduces API for AbstractComponent, removing a rarely-used feature (supports Save All, currently only relevant to Scenario plugin)
* Simplifies solutions for nasa/MCT-Plugins#72 by permitting explicit identification of objects to be included in the Save All action (in this case, activities found by way of collections)

 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: Save All
 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?