Add group system

Initial commit, not tested in multiplayer yet. Some revisions still left
to do.

SQF/dayz_code/actions/AdvancedTrading/functions/z_at_fillCategories.sqf

@@ -5,5 +5,5 @@ call Z_clearBuyList;
 
 {
 	_index = lbAdd [Z_AT_BUYABLELIST,_x select 0];
-	lbSetPicture [Z_AT_BUYABLELIST,_index,"\z\addons\dayz_code\gui\epoch\arrow_right.paa"];
+	lbSetPicture [Z_AT_BUYABLELIST,_index,"\z\addons\dayz_code\gui\grey\arrow_right.paa"];
 } forEach Z_traderData;