Shorten sockets file path
Use Qt dedicated 'runtime' standard location for sockets.
This fix an issue on macos where temporary directories are so long
that a user having a long nick would bypass the socket file path length
Reviewers: dfaure, dvratil
Reviewed By: dfaure, dvratil
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D21650