bind mouse capture to middle mouse button

pretty standard across emus that do this upon hotkey

Author: vadosnaprimer

src/BizHawk.Client.Common/config/Binding.cs

@@ -66,7 +66,7 @@ void Bind(string tabGroup, string displayName, string defaultBinding = "", strin
 			Bind("General", "Toggle Messages");
 			Bind("General", "Toggle Display Nothing");
 			Bind("General", "Accept Background Input");
-			Bind("General", "Capture Mouse");
+			Bind("General", "Capture Mouse", "WMouse M");
 
 			Bind("Save States", "Save State 1", "Shift+F1");
 			Bind("Save States", "Save State 2", "Shift+F2");