AADL Navigator Does not Properly Support Working Sets

[philip-alldredge]

• Selecting Top Level Elements->Working Sets with a working set selected results in only projects that aren't included in a working set appearing.
  • Working sets do not appear because the org.eclipse.ui.navigator.resources.workingSets contentExtension isn't enabled.
• Unable to add Plugin_Contributions to working set.

child of #1371

[lwrage]

This will be implemented as part of the scalability task.

[lwrage]

• Follow same approach as PDE project explorer.
• Add categories for sorting.

[lwrage]

The AADL navigator implementation is a bit of a mess, in particular the AadlNavigatorLabelProvider is wrong. After fixing it the order of entries doesn't change randomly any longer (see #1120).