Remove swt/gef editors which have been replaced by JavaFX editors

In order to prevent from unwanted side-effects, the SWT editors will be remved from plugin.xml if there is a JavaFX implementation for them. Similiarly the SWT Model-Editor View on the right will be removed.

(from redmine: issue id 3938, created on 2020-03-03, closed on 2020-03-26)