Add notebook features

This one adds two useful features to the Notebook widget:

* Icons in Tab headers: makes it possible to add an icon in front of a tab label
* Expandable Tabs: if enabled, expands the tab lables

TODO: MAC backend support (I think the icons have to be drawn manually and there seems to be no native support for expandable tab labels).
TODO: WPF expands the tabs only on initialization and resize. Toggling the property after the widget has been shown needs a resize to take effect. Maybe someone with WPF knowledge can fix that easily? The widget should be forced to recalculate children sizes and to redraw. Simply invalidating Measurement doesn't help here.