[platform-ui-dev] Handling Empty Window Working Sets

[Kimberly Horne <kim@xxxxxxxxxxxxx>]

As per the discussion in https://bugs.eclipse.org/bugs/show_bug.cgi? 
id=113832 we've come up with a strategy for handling the usage of  
empty window working sets.

If we're currently using an aggregate working set (such as the window  
set) for our view and that aggregate is empty we act as if we're not  
using any working set at all.  We continue to maintain knowledge of  
the working set and will react to elements being added to or removed  
from the set as they occur.  eg: should the set become empty we then  
act as if we are using no working set and when an element once again  
populates the set we then filter on it again.

There is a patch in the previously mentioned bug that illustrates  
what we did to make this happen for ResourceNavigator.  The patch is  
quite small and the logic straightforward.

We're recommending that all SDK views that utilize working sets  
follow this strategy.  Javadoc will be added to  
WorkingSetFilterActionGroup and IWorkbenchPage describing this  
strategy later today.