Fixed calculation of natural size
Fixed calculation of natural size

The natural size is now stored in a field in the widget backend, so if
a GetPreferred* method is called but the measure is not invalidated,
we can return a value for natural size (the min size is always stored
in DesiredSize).

sevoku/xwt,iainx/xwt,residuum/xwt,directhex/xwt,mminns/xwt,antmicro/xwt,steffenWi/xwt,mminns/xwt,mono/xwt,akrisiun/xwt,lytico/xwt,cra0zy/xwt,TheBrainTech/xwt,hamekoz/xwt,hwthomas/xwt