Operation doesn't check for update if selection in layer view didn't changed

.. but other preferences the PropertyValues checks for. It's a problem everybody could run into if extending udig. the effect can described as follows:

• i created an operation, used the net.refractions.udig.ui.operation extension point to get an Action for the targetClass net.refractions.udig.project.ILayer
• subclassed AbstractPropertyValue and created my own PropertyValue class (checks preferences for layer using a preferences store)
• the user select a layer, changed the preference for the selected layer and as a result an operation should be available within the context menu of the still selected layer
  BUT the action (OpAction) is not visible (PropertyValue would return true) because the selection didn't changed.
  If the customer deselects the layer and selects it again it works.

But it is possible to run into performance issues if PropertyValue implementations are very heavy.

see patchfile