More rewrites/fixes:

More rewrites/fixes:
- Added check in addExtensionSql() if $sql is empty and if so, return without
  adding empty entries
- Added saving of 'previous_extension' which is the "source extension" of the
  dependency request ('previous_extension' requires other one)
- Added check for entry in 'previous_extension' and abort (under development)
- Added search_array() from PHP user comments (thank you for sharing this code)
- Other minor fixes/improvements
- TODOs.txt updated