qml: add fallback widget for player toolbar

Instead of using spacer widget which is invisible, use a focusable text based dummy control which properly indicates the unknown control.

This is helpful when checking out an older branch that has the unimplemented control after saving the player toolbar configuration in the branch that the control exists.

It is still possible to move the unknown widget between views in toolbar editor. However, if the unknown widget is deleted in toolbar editor, it is not possible to add it back. The only way to add the lost widget back is to manually edit the interface configuration file.