make GetDefinition work on string parameter

This can be useful to check if a definition is loaded, ideally the implementation is moved to the engine.

planet/System.ocg/Definition.c

@@ -0,0 +1,29 @@
+/*--
+		Definition.c
+		Authors: Maikel
+
+		Functions generally applicable to definitions.
+--*/
+
+static GetDefinition_Loaded_Definition_List;
+
+// Returns the definition or nil if par is a string and the definition exists.
+// See the documentation for the case when par is an integer.
+global func GetDefinition(par)
+{
+	// Overload behavior when par is a string.
+	if (GetType(par) == C4V_String)
+	{
+		if (GetDefinition_Loaded_Definition_List == nil)
+		{
+			// Fill the static list of definitions when is has not been generated yet.
+			GetDefinition_Loaded_Definition_List = {};
+			var i = 0, def;
+			while (def = GetDefinition(i++))
+				GetDefinition_Loaded_Definition_List[Format("%i", def)] = def;
+		}
+		// Return the definition if in the list and nil otherwise.
+		return GetDefinition_Loaded_Definition_List[par];
+	}
+	return _inherited(par, ...);
+}