Changed external reference flag texts to obey current name/label mode

Text shown in external reference type diagram flags now obeys the
currently selected Name/Label model that can be controlled using code in
class org.simantics.NameLabelUtil.

Previously the text shown in normal diagram external reference flags was
always preferring to show the Label property of diagrams and related
modules and if labels were not defined, the Name property would be used
instead.

refs #7178

Change-Id: I85efb6e68f92fdb18d1445037da5285700ca5398