TextEditor: Reset editor width when disabling preview mode

When disabling preview mode, reset the fixed width of m_editor so that
it fills out the window again even after resizing the splitter.

Userland/Applications/TextEditor/MainWidget.cpp

@@ -735,6 +735,7 @@ void MainWidget::set_preview_mode(PreviewMode mode)
         update_markdown_preview();
     } else {
         m_no_preview_action->set_checked(true);
+        m_editor->set_fixed_width(-1);
         set_web_view_visible(false);
     }
 }