Added 'sell all' tooltip to show items you can sell

* Now 'sell all' button with enable/disable
* Fixed display issue with 'sell all' button
* Also changed spinbox button click

modules/game_npctrade/npctrade.otui

@@ -250,6 +250,7 @@ MainWindow
     anchors.right: next.left
     anchors.bottom: parent.bottom
     margin-right: 10
+    visible: false
     @onClick: modules.game_npctrade.sellAll()
 
   Button