Add documentation to the styleName property of the VirtualKeyboardSettings

Most of the embedded Qt apps most likely want to use their own custom style and maybe also want to let the user change the keyboard style themself. For this it would be great if there would be an option within the KeyboardSettings to set the current active style. This is already there, but currently marked as internal. This should be an official feature. Also building the keyboard without including a style should be an option.