Handles the "Copy a template from the theme this is based on" action.

• Accessed at index.php?action=theme;th=xx;sesc=session_id;sa=copy
• Will only let you copy a file that doesn't already exist in the selected theme's directory
• If no base theme is specified for this theme, looks for files in the default theme
• Copies the template specified by "template" if it doesn't already exist (ie action=theme;th=xx;sesc=session_id;sa=copy;template=Template)
• Also lets you copy a language file by specifying a "lang_file" in the URL (action=theme;th=xx;sesc=session_id;sa=copy;lang_file=Language.file), although there's currently no interface for that