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/mouse/wrap_Mouse.cpp

@@ -57,7 +57,7 @@ int w_getSystemCursor(lua_State *L)
 	Cursor::SystemCursor systemCursor;
 
 	if (!Cursor::getConstant(str, systemCursor))
-		return luaL_error(L, "Invalid system cursor type: %s", str);
+		return luax_enumerror(L, "system cursor type", str);
 
 	Cursor *cursor = 0;
 	luax_catchexcept(L, [&](){ cursor = instance()->getSystemCursor(systemCursor); });