Don't set modified flag for database when attaching/detaching database

Don't set the modified flag of the main database if we're attaching or
detaching another database since these actions don't alter the original
database.

See issue #1249.

src/MainWindow.h

@@ -143,6 +143,8 @@ private:
         UpdateStatement,
         DeleteStatement,
         CreateStatement,
+        AttachStatement,
+        DetachStatement,
         OtherStatement,
     };