[generator] Don't trim 8 name chars on non-set-listeners. (#181)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=58403

An event names is generated from the method name which was supposed to be
`setOnXxxListener()`. Though we had been generating events for methods that
do not end with "Listener", resulting in weird event names (e.g. it was
"VisibleWidt" for `setOnVisibleWidthChanged` method).

Trim trailing 8 characters only on `setOnXxx**Listener**()` methods.

tools/generator/Method.cs

@@ -396,7 +396,8 @@ internal string CalculateEventName (Func<string, bool> checkNameDuplicate)
 		{
 			string event_name = EventName;
 			if (event_name == null) {
-				event_name = Name.Substring (0, Name.Length - 8).Substring (3);
+				var trimSize = Name.EndsWith ("Listener", StringComparison.Ordinal) ? 8 : 0;
+				event_name = Name.Substring (0, Name.Length - trimSize).Substring (3);
 				if (event_name.StartsWith ("On"))
 					event_name = event_name.Substring (2);
 				if (checkNameDuplicate (event_name))