Currently one needs to first obtain the PaltformUI Workbench and then
call getSharedImages on this instance. As this actually maps down to
WorkBenchPlugin that do not require a running workbench this limits the
usage of such code only wanting to share images.

This adds a new ISharedImages.get() that directly calls the code the
IWorkbench implementation would call and replace static calls to the
workbench with this one.