Fix possible memory corruption when creating dialogs with selectors.

Fix possible memory corruption when creating dialogs with selectors.

The PsppireSelector widget's class structure contains a map of selectors
indexed by the source widget with which they are associated.  Until now, that
map was never depopulated.   This could have caused problems if newly created
widgets happened to use the same address as old, destroyed ones.  This change
removes selectors from the map upon destruction.