Non-Qt local socket communication on Unix systems.

The new class Socket handles thread-safe unix communication now. On
windows systems the old code is used.
The server code still uses the QLocalSocket class since each connection
has its own thread anyway. Porting is not as important.

This allows us to use a single thread on the client side and avoiding all
that ugly and error-prone code that tries to maintain one socket per
client thread.
The result is less crashes, lower resource consumption and cleaner code
(not counting the #ifdefs).