fix ticket #1043 - rename defaultconfig.ini.php of a project - http://developer.jelix.org/ticket/1043
drop this unnecessary fix - will be done alone
defaultconfig.ini.php is renamed to mainconfig.ini.php

changes merge from Foxmask's branch
New jApp::mainConfigFile() method to retrieve the path of the main configuration file

It is a solution to avoid this deprecated_in_jelix_1.5.php file.
Removed deprecated_in_jelix_1.5.php file