Shorten sockets file path

Summary:
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
of 104chars.

Reviewers: dfaure, dvratil

Reviewed By: dfaure, dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21650