#15 call graph: list/filter list possible improvements

[Yann Dirson]

It would be useful to have items already visible
flagged as such in the list (eg. as selections in a
multi-selection list).
That would however possibly be inconsistent with items
being removed from the list when selecting "remove
item". However, there is already an inconsistency wrt
this kind of removal: when you re-open the list after
removing some items, the removed items are back again
(which is a problem, as because of this you cannot use
the filter/list twice to refine a graph: pressing OK
will discard any actions done in previous filtering
dialog)