InputConfigDiag: Use "..." for complicated expressions

The full expression is quite often too big for a simple button
label, so encourage people to use the full editor to edit it.

Source/Core/InputCommon/Src/ControllerInterface/ControllerInterface.h

@@ -73,6 +73,13 @@ public:
 				return 0;
 		}
 
+		bool IsComplicated() {
+			if (parsed_expression)
+				return parsed_expression->is_complicated;
+			else
+				return false;
+		}
+
 	protected:
 		ControlReference(const bool _is_input) : range(1), is_input(_is_input), parsed_expression(NULL) {}
 		ciface::ExpressionParser::Expression *parsed_expression;