Adding $mode to group canView($mode) calls, was missing from some, so
groups idden in form view were getting hidde in list view as well.
Removed unnecessary language folders
Revert "Adding $mode to group canView($mode) calls,... and
modified form.php and group.php so that if group is not shown then the form really doesn't try to update data and if shown only in details then really shown in details + in list view.

This reverts commit a3c91c0b660f1077f06d84d88f63737c4ac24912.
Restored Hugh's changes (commit a3c91c0 ) that I reverted in previous commit (in vain). Yes, they didn't work as expected, but the cause seems to be that the changes in form.php and group.php were missed.
The main goal is that if a group is not shown in form view then the form really doesn't try to update group's data and if in "details view only" mode then really shown in details + in list view). Although it was achieved without these modifications (in list.php, view.base.php and view.raw.php), they may be still necessary in some (unforeseen by me) situations (sorry, Hugh).
Merge branch 'joomla3_hidden_groups' into joomla3