Refactor the Actions in EditorTopComponent (com.sun.hotspot.igv.view.actions). Move Action specific code from EditorTopComponent to the corresponding Action.

Fix some Bugs in the EditorTopComponent Actions:
- ShowEmptyBlocksAction is selected by default but only enabled in CFG view: don't select it when disabled (greyed out)
- Only enable ExtractAction, HideAction and ShowAllAction when they can have an effect
- ReduceDiffAction gets is not enabled and we get stuck when difference graph selection is >1 and it includes the last graph of the group