Reduce number of refreshes and memory usage of cross references widget.

* Generates IL for cross references only when visible instead of all at once.
* Releases memory for IL after the cross references retrieves the
  tokens.
* Much faster algorithm for updating IL when functions change.
* Don't refresh cross references at all if the widget isn't visible.
  Retreiving the new set of cross references is deferred until the
  widget becomes visible again.
* Don't redo the query for the list of cross references when anything in
  the entire UI changes, even the window layout. Only recompute the list
  when an event for a new cross reference selection comes in.