Constants removed: THEME and SMARTY_SPL_AUTOLOAD

[ THEME ]

Replaced constant THEME by configuration 'Theme' key

The theme is now:

• indicated in the configuration, not in index.php for the default one ;
• a parameter of Context::initializeTemplateEngine($theme).

[ SMARTY_SPL_AUTOLOAD ]

Dropped SMARTY_SPL_AUTOLOAD constant, not needed, the Smarty
default behavior is to honor existing SPL autoloaders.

This is a follow-up of commit e34632ba6b36d0290b9af3acd5502c90af8ff54b.