Fix PageControl bug where left or right tabs are unresponsive to clicks
Fix PageControl bug where left or right tabs are unresponsive to clicks

PageControl tabs were on the left or right, and you went over a certain
number of tabs, then you couldn't change tabs by clicking with the
mouse.

The TabSheetAtPos() never took into account the TabPosition, and it
always used MaxButtonWidthSum, which is only meant for Top or Bottom
postitions.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI