Add handling of magazine overflow in AdvTrade and free slot icons

- Added proper handling of pistol mags and regular mags in gear to
prevent magazine overflow
- Added pre-check if return change will overflow gear + backpack free
space. If change will not fit in gear + backpack free space then buy is
prevented with a message notifying the player. In the case of a sale it
proceeds anyway, but notifies player if overflow occurs (see comments in
returnChange)
- Consolidated duplicate code for calculating free space and returning
cargo of an object into new calcFreeSpace
- Old calculateFreeSpace renamed to displayFreeSpace
- Added Weps/Mags/Bags icons to free slot numbers display

SQF/dayz_code/Configs/RscDisplay/advancedTrading.hpp

@@ -393,10 +393,10 @@ class AdvancedTrading
 			colorBackground[] =  {1,1,1,1};
 			color[] = {0,0,0,1};
 		};
-		class RscText_ATT8: ZSC_RscText
+		class RscText_ATT8: ZSC_RscStructuredText
 		{
 			idc = 7404;
-			text = "0/0/0";
+			text = "";
 			x = 0.45 * safezoneW + safezoneX;
 			y = 0.32 * safezoneH + safezoneY;
 			w = 0.15 * safezoneW;