[FileMetaDataProvider] support symlinks with a new "Link to" item

Summary:
Add information about symbolic links to FileMetaDataProvider
with a new "Link to" ("kfileitem#linkDest") item

Dolphin tool-tip before:
{F7864287, size=full}

Dolphin tool-tip after:
{F7867842, size=full}

Reviewers: #baloo, ngraham, meven, bruns

Reviewed By: #baloo, ngraham, meven, bruns

Subscribers: meven, bruns

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D26469