Make liveEditHelper cache calls to isDocumentLiveEditable at event level

isDocumentLiveEditable are expensive since they include a call to documentManager.hasPermission (to check WRITE_PROPERTIES). Since liveEditHelper.isDocumentLiveEditable is to be called 6 times for each occurence in a single "rendered" JSF attribute it makes sence memoizing the result of the first call in the Event Seam context (e.g. as a field of the Event scoped liveEditHelper seam component).