search term for users and groups may occur anywhere in the name or displayname, not just in the beginning

* test case added to avoid adding them later

Some things my IDE complained about while creating a custom backend.

Fix PHPDoc and typos

Some things my IDE complained about while creating a custom backend.

Conflicts:
	lib/private/group/database.php
	lib/private/group/dummy.php
	lib/private/group/example.php

backends. This is a port to master from PR #7745

remove OC_GROUP_BACKEND_GET_DISPLAYNAME option for group backends

Conflicts:
	lib/private/group/backend.php

LDAP: getDisplayNamesInGroup is not an option for group backends anymore

Conflicts:
	apps/user_ldap/group_ldap.php
	apps/user_ldap/group_proxy.php

clean up group backends

Conflicts:
	lib/private/group/database.php

remove now unnecessary test

implement getDisplayNames in group manager

adjust user manager tests

test for group manager's displayNamesInGroup

trim must not be used in empty in PHP < 5.5

keep the constant to not provoke PHP warnings

Conflicts:
	lib/private/group/backend.php

Conflicts:
	lib/private/vcategories.php

* fix typo in OC_Group_Database::DisplayNamesInGroup's SQL clause
* check array_diff return value in OC_Group::displayNamesInGroups,
  when there is no difference between two arrays, it will return
  NULL, so we have to take care of it.