[NbtkStyle] Support fetching of CSS properties of type `unsigned int'.
[NbtkStyle] Support fetching of CSS properties of type `unsigned int'.

NbtkScrollView's xthickness and ythickness style properties are
of type `unsigned int'. Handling of pspecs of that type was not
supported in NbtkStyle, so the values set in CSS were not applied.

jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx