Improve code quality / code refactoring

[MightyMCoder]

• centralize SQL-commands (in common_function.PHP), e.g.: read users (possible keepers) from adm_users list
• consistent usage of global variables (e.g. instead of $GLOBALS['gCurrentUser'] use "global gCurrentUser")
• create "same" code structure in folders fields and items (could be particular modular I guess)
• extend common_function.php by modularisable code blocks from different files (e.g. loops to check things)
• improve in file code documentation