grid: introduce an overloaded GetClientRect() function.

This takes into account BorderStyle and border widths from the theme
manager.
grid: First run through code removing hard-coded border widths.

This was mostly the replacement of FMargin field variable usage. Tested
and grid still works like before.
grid: minor code formatting fix.
grid: Removes last traces of that pesky FMargin variable.
grid: Final piece of the puzzle. BorderStyle is now fully working.
Fixes small spelling mistake in code comment.
Merge branch 'gridborderstyle' into develop