Moved system language file.
Removed unneeded system data files.
Moved system images.
Moved common styles.
Removed backend module transfer section file.
Section handler: Improved section handling.

Removed MainSectionHandler class as there's no longer a need for system
section file. Also added option for transfering data to backend module
without requiring backend_module.xml.
Removed empty scripts directory.
Base module class: Changed from double quotes to single.
Language handler: Load system language file from right path.
Module handler: Add support for site-specific modules.
Moved system templates.
Separated config to system and site.
Main index file: Load site config if present.
Page info: Change lookup paths for files.
Gitignore: Ignore `site/` directory.