Refactor ACF module structure

[herrvigg]

Redesign the ACF module around two classes :

• QTX_Module_Acf_Register: set up the derived fields (front and admin)
• QTX_Module_Acf_Admin: handle the admin sections and settings, loaded conditionally (admin only)

Other changes:

• Delete internal QTX_Module_Acf_Interface
• Remove all references to "plugin" instance
• Module entry point handles only the main init() (for plugin and theme as before)
• Rename all derived fields with QTX_Module_Acf_Field prefix
• Comment dead code (already unused in legacy plugin), to be sorted out.

All changes are supposed to preserve all the current functionalities.