[BUG #7707] qx.ui.mobile.form.renderer.Single doesn't hide the label of a multi-line widget

* Refactored the methods addInSeparateLines() and addInOneLine(). Merge them to one method "_addRow()" which gets the layout as a parameter.
* The display as one or two line widgets is now handled via CSS, through the occurrence of VBox beside the "form-separation-row".
* addInSeparateLines() and addInOneLine() are now deprecated.