[NbtkTable] ensure col span is taken into account for preferred child height
[NbtkTable] ensure col span is taken into account for preferred child height

This makes sure the entire cell width is included in the for_width
parameter for each child when the table is calculating it's preferred
height.

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