Add sell all button to NPC trade

modules/game_npctrade/npctrade.otui

@@ -243,6 +243,15 @@ MainWindow
       checked: true
       @onCheckChange: modules.game_npctrade.onShowAllItemsChange()
 
+  Button
+    id: sellAllButton
+    !text: tr('Sell All')
+    width: 64
+    anchors.right: next.left
+    anchors.bottom: parent.bottom
+    margin-right: 10
+    @onClick: modules.game_npctrade.sellAll()
+
   Button
     id: tradeButton
     !text: tr('Buy')