Import KStatusBarOfflineIndicator from KIO

Summary:
KTorrent is the only user of this class and it doesn't really belong into KIO. Importing it here will allow to get rid of it in KIO.

See T12373

Reviewers: guisson, stikonas, shaforostoff

Reviewed By: stikonas

Maniphest Tasks: T12373

Differential Revision: https://phabricator.kde.org/D26411