Mantis#1469. Thank you kindly, Mikem for a patch that addresses:

Currently LSL code such as below does not compile on OpenSim, but compiles fine in Second Life:
list mylist = [];
mylist += [1, 2, 3];
mylist += "four";
list newlist = mylist + 5.0;
The problem is that the LSL_Types.list class does not have an operator for adding a string to a list.
I am including a patch which implements adding a string, integer or float to a list.
I am also including tests. The file LSL_TypesTestList.cs belongs in 
OpenSim/Tests/OpenSim/Region/ScriptEngine/Common/.

OpenSim/Region/ScriptEngine/Common/LSL_Types.cs

@@ -425,6 +425,30 @@ namespace OpenSim.Region.ScriptEngine.Common
                 return new list(tmp);
             }
 
+            private void ExtendAndAdd(object o)
+            {
+                Array.Resize(ref m_data, Length + 1);
+                m_data.SetValue(o, Length - 1);
+            }
+
+            public static list operator +(list a, string s)
+            {
+                a.ExtendAndAdd(s);
+                return a;
+            }
+
+            public static list operator +(list a, int i)
+            {
+                a.ExtendAndAdd(i);
+                return a;
+            }
+
+            public static list operator +(list a, double d)
+            {
+                a.ExtendAndAdd(d);
+                return a;
+            }
+
             public void Add(object o)
             {
                 object[] tmp;