Change lang folder name to languages.

Update files path for text domain.
Move the admin pages to the includes folder.
Move class.message,php to admin folder.
Move class.filesystem.php to admin folder.
Move class.capabilities.php to admin folder.
Move class.admin-actions.php to admin folder.
Move CSS and JS and images to assets folder.
Move inc.processes.php to admin folder.
Fix filter by character and pagination to use the admin action class.