Define translated constants after loading textdomain

[rwngallego]

This defines some of the constants that require translated text after the textdomain is loaded to remove the warning:

Notice: Function _load_textdomain_just_in_time was called incorrectly. 
Translation loading for the perfecty-push-notifications domain was triggered too early.
This is usually an indicator for some code in the plugin or theme running too early.
Translations should be loaded at the init action or later.

[rwngallego]

This is a refined fix of PR #207.