grid: introduce an overloaded GetClientRect() function.
grid: introduce an overloaded GetClientRect() function.

This takes into account BorderStyle and border widths from the theme
manager.

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