tabbar:
new presentation element as a child for ptabcontainer

needs number of tabs/children, margin (?) and total width (minimal)

returns dimensions

if child is non-empty then show it, with margin around it. 

Width: calculate the height/width of all non-empty tabs
calculate the rest to sum up to the total width (but it should be at least a
minimum value, maybe 2-4x the height ?) 

vertical placement based on baseline.
horizontal placement: center

special draw: strings as symbols (normal text)
              (there will be no parsing anyway)

check if OMSTR is supported