TextEditor: Use more direct approach for fallback IEditor construction

There are still a few places where the EditorWidget is the primary
object, or several of them per "visible" IEditor (e.g. Diff).
Provide a means to streamline setup there, too.

Change-Id: I14cfbd68e555ebc539e707032a0e5bef563e0a36
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/diffeditor/diffeditorplugin.h

@@ -35,7 +35,6 @@
 #include <extensionsystem/iplugin.h>
 
 namespace DiffEditor {
-
 namespace Internal {
 
 class DiffEditorPlugin : public ExtensionSystem::IPlugin