Added option to enable the user define the grid plane size/amount.

NOTE: It was add a new option to the Preferences dialog where the user can define the grid size/amount. [Micheus]
The range is 10 (current) to 200 and the camera distance (for reset camera operation) is calculated to 80% of this size (like the current too).

* ref. this thread  (with screen shot): http://www.wings3d.com/forum/showthread.php?tid=552&pid=3886#pid3886