Check that we don't clobber a custom bitmap in the dialog.

The (non-obvious) trick is that the name of these is non-empty, so we
can check that to filter out pool default items that Draw likes to put
into item sets (?), as well as just plain weird items that Draw likes to
put into item sets, while avoiding relying on the surprisingly
implemented ImpGraphic::operator==().

(regression from 38d0047d)

Change-Id: I0b94e49ef3a9a32c188c3b117a57f780f55e1584