Option to set border width of PanelItembar widget.

An option to set the border size of the PanelItembar widget, since there is currently no way of setting the space between the window borders and the panel plugins themselves except for settings smaller icon size manually, and that doesn't even work if the plugin doesn't use it. It can't be done even in CSS. Setting padding/margin both on .xfce4-panel and .xfce4-panel:first-child (the PanelItembar widget) doesn't do anything, and border properties don't actually add space around the children. So basically this is the desired effect, on the right you can see the wnck pager plugin witch would normally be exactly filled to panel end: