Fixed memory leak with String objects

The member _godot_string should never be straight out overwritten ever without
first destroying the underlying string object's memory. This change solves the
problem through the introduction of a new private constructor to create String
objects with a pre-existing godot_string handle.

include/core/String.hpp

@@ -29,6 +29,9 @@ public:
 class String {
 	godot_string _godot_string;
 
+	String(godot_string contents) :
+			_godot_string(contents) {}
+
 public:
 	String();
 	String(const char *contents);