Expose group information to Dolphin's Information panel, tooltips, etc

Summary:
Expose Group information for files and folders so that it can be shown in Dolphin's Information panel, tooltips, etc.

FEATURE: 308002
FIXED-IN: 18.08.0

Test Plan:
Deploy change, create new user, open Dolphin, open Information Panel
- Group is not visible by default, as expected
- When made visible, Group appears between User and Permissions, as expected

Deploy change, use existing user account, open Dolphin, open Information Panel
- Group is not visible by default, as expected
- When made visible, Group appears between User and Permissions, as expected

{F5783397}

Reviewers: #baloo, michaelh

Reviewed By: michaelh

Subscribers: cfeck, anthonyfieroni, zzag, michaelh, broulik, #dolphin

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D11897