- undefine PUCLASS_LIST, as we have our own (old_warning--)

- undefine PUCLASS_LIST, as we have our own (old_warning--)
- PropertyObject: remove additional "values" member again
- don't create entry lists for <list>, <select>, and <combo> at dialog
  creation and delete them on dialog close, but let a separate class
  fgValueList handle this. The three widgets are now subclassed and the
  derived widget lets fgValueList manage the lists
- make <select> consistent with <list>. This breaks backwards
  compatibility, but not a single dialog in fgfs uses it and did so
  since ... forever. (Shoot me!)

Rationale: now that dialogs are a bit more dynamic than they used to be
(thanks to embedded Nasal), we have to delete and recreate entry lists
during dialog use. Deleting only at dialog closing doesn't cut it anymore.
Especially list widgets that are updated several times per second would
use up a lot of memory otherwise. The route-manager doesn't update that
often, but it did already leak. One TODO less.  :-)