Fix resource selection dialog not disappearing

Clicks to reproduce:
1) warehouse
2) buysell tab
3) any slot
4) any other tab of the warehouse tabs
5) deselect warehouse
The resource selection used to still be there, with no way to remove it

horizons/gui/tabs/buyselltab.py

@@ -111,6 +111,12 @@ def show(self):
 		self.refresh()
 		ExtScheduler().add_new_object(self.refresh, self, run_in=0.4, loops = -1)
 
+	def is_visible(self):
+		# this tab sometimes is made up an extra widget, so it must also be considered
+		# when checking for visibility
+		return super(BuySellTab, self).is_visible() or \
+		       (self.resources is not None and self.resources.isVisible())
+
 	def _refresh_trade_history(self):
 		self.trade_history.removeAllChildren()
 		unused_rows = set(self.trade_history_widget_cache.keys())