better events for get_single_uid/cid

While working on improving the extensibility of the contextmenu plugin particularly with attention to the zipdownload plugin I found the need for a plugin to be able to override the result of the get_single_uid and get_single_cid functions. In my case, so I can return the id of the item on which the context menu was triggered rather than any current selection.

Updated with Thomas' suggestion from #231