Also merge Doxygen comments for overloaded constructors in Python.

This was done for plain functions/methods but not constructors, which don't
have "kind=function" attribute.

Just don't check for "kind" at all, the presence of "sym:overloaded" should be
good enough to indicate that it's an overloaded something.

Also add a test for overloaded constructors documentation.

Examples/test-suite/doxygen_parsing.i

@@ -39,6 +39,12 @@ int someVar=42;
 class SomeAnotherClass
 {
 public:
+	/// First overloaded constructor.
+	SomeAnotherClass(int) { }
+
+	/// Second overloaded constructor.
+	SomeAnotherClass(const char*) { }
+
 
 	/**
 	 * The class attribute comment