Don't clear buying list on category switch + AdvTrade fixes

The buying list now acts like a shopping cart. It is no longer cleared
when switching between categories or containers. Closing the dialog or
switching to sell mode still clears it.

Also fixed:
- Filter button in sell mode
- Bug where filtering sellable list readded items already moved to
selling list
- Visual price display not clearing when remove all button is used on
buying list
- Gear container picture showing rank instead of portrait in buy mode
- Dialog is no longer closed when trying to sell zero items
- LbCurSel is now reset when the buyable list is cleared
- Category is no longer opened onLbSelChanged. Instead you need to
double click or use the "View" button, so the arrow keys can be used for
selecting too.

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

@@ -4,7 +4,7 @@
 *	Calculate the total price for single currency.
 **/
 private ["_sellPrice","_ctrltext"];
-#include "defines.sqf";
+#include "defines.hpp"
 
 _sellPrice = 0;