Tables: more consistent use of CellPadding.x*2 and clip-rect on right-most side of non-bordered column + fix cellbg for standalone TableHeader call.

Using CellPadding.x on both sides when BorderV is off, generally most consistent and with default value (4,2) promotes at-glance visible spacing between non-bordered columns. Effectively double horizontal padding on non-bordered columns.
Made ClipRect.Max.x matches WorkMaxX which is where we'd like to go for windows themselves.
TableHeader() submit single cell bg color if not already submitted as a full header row.
Misc comments/docs updates.

docs/FAQ.md

@@ -214,8 +214,6 @@ Button("OK");          // Label = "OK",     ID = hash of ("MyOtherWindow", "OK")
 End();
 ```
 
-We used "..." above to signify whatever was already pushed to the ID stack previously:
-
 - If you have a same ID twice in the same location, you'll have a conflict:
 ```cpp
 Button("OK");