Refactor core file structure and init loaders

[herrvigg]

The core and utils files are too large and difficult to maintain.
Hooks are created while loading some PHP files which is hard to control.

Break core and utils into smaller pieces:

• init.php - the main plugin loader from core that is qtranxf_init_language()
• language_config.php - language entries in $q_config from utils
• language_detect.php - detect and redirect, mostly from core
• rest_api.php
• url.php - most of url_info handling from core and utils

Refactor qtranxf_init_language() to make some init calls more explicit.
Refactor core/hooks/frontend loaders to move all creations of hooks into functions.
Deprecate legacy qtranxf_init (no-op).