The phpbb_user object might be used for language variables. Pass it as a
function argument to prepare_form_acp() instead of using globals.
PHPBB3-10018
Remove the $request property from the phpbb_avatar_driver class and rather
pass it as function argument if it's needed in a function. Currently this
is only the case for the class methods prepare_form() and process_form().
PHPBB3-10018
The upload avatar driver will now inform the user if insufficient data has
been entered for both remote and local avatar uploads.
The local avatar driver (gallery avatar) will also inform the user if he
didn't select a category and/or file before submitting.
PHPBB3-10018
The addition of "core_" that was used until now would require additional
avatar drivers (e.g. in extensions) to also include that in their class
name which would be incorrect.
PHPBB3-10018
