define URL constants later

Hello @chouby,

thanks for your work on this really nice plugin.

I’d like to suggest some small improvement or bugfix.
I realized, that by definig some constants really early, like you do here in polylang.php, it’s impossible to filter the used *_url functions.

// our url. Don't use WP_PLUGIN_URL http://ww.wp.xz.cn/support/topic/ssl-doesnt-work-properly
define('POLYLANG_URL', plugins_url('', __FILE__));
// default url to access user data such as custom flags
if (!defined('PLL_LOCAL_URL'))
	define('PLL_LOCAL_URL', content_url('/polylang'));

But if you move only theese two defines, down into your existing

protected function define_constants()

everything works fine, also for people with modified plugins_url and content_url.

https://ww.wp.xz.cn/plugins/polylang/