[ticket/13358] Add file_downloader as replacement for get_remote_file

PHPBB3-13358
[ticket/13358] Add file_downloader to version_helper

PHPBB3-13358
[ticket/13358] Fix tests and use exceptions instead of user object

PHPBB3-13358
[ticket/13358] Do not pass variables by reference

PHPBB3-13358
[ticket/13358] Use protected and public keywords where applicable

PHPBB3-13358
[ticket/13358] Correctly create version_helper in acp_extensions

The full namespace for the file_downloader is now also being used in
version_helper.

PHPBB3-13358
Merge pull request #3157 from marc1706/ticket/13358

[ticket/13358] Add class for retrieving remote file data