Preference for preferring External Editor for opening external documents stored in the database

Currently "External Editor" adapter does not have any priority over e.g. Text Editor contributions coming from Simupedia which means that any Office-documents imported into models (org.simantics.document[.ui]) are generally opened in a useless Text Editor by default.

The purpose of this issue is to add an Eclipse preference that causes ExternalEditorAdapter to re-configure its priority so that it is definitely always preferred.