Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/image/wrap_Image.cpp

@@ -50,7 +50,7 @@ int w_newImageData(lua_State *L)
 		{
 			const char *fstr = luaL_checkstring(L, 3);
 			if (!getConstant(fstr, format))
-				return luaL_error(L, "Invalid pixel format: %s", fstr);
+				return luax_enumerror(L, "pixel format", fstr);
 		}
 
 		size_t numbytes = 0;