This listens to changes to the current selection, and propagates them through
the ISourceProvider framework (a common language in which
events are communicated to expression-based services).
A static class linking the names of variables in an IEvaluationContext to the
priority they should be given when doing conflict resolution.
In the future, it will possible to define a new variable (i.e., piece of
application state) that you want to use inside of the
org.eclipse.ui.contexts, org.eclipse.ui.handlers
or org.eclipse.ui.menus extension points.