EditorManager: Use static pattern, adjust surrounding code

Change-Id: I3255a0150cd9a730336456c5a9f986eb74fefbff
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/dialogs/readonlyfilesdialog.cpp

@@ -267,7 +267,7 @@ int ReadOnlyFilesDialog::exec()
             }
             break;
         case RO_SaveAs:
-            if (!EditorManager::instance()->saveDocumentAs(d->document)) {
+            if (!EditorManager::saveDocumentAs(d->document)) {
                 failedToMakeWritable << buttengroup.fileName;
                 continue;
             }