wait for seaf-daemon to quit and remove the related warnings

- allow 50ms plus 10ms for `ccnet` and `seaf-daemon` to quit before sending `TERM` signal
- don't print the silly warnings like
```
[12/26/2014 16:14:39]QProcess: Destroyed while process is still running.
[12/26/2014 16:14:39]QProcess: Destroyed while process is still running.
```
We already call `daemon->kill()` in `SeafileApplet::onAboutToQuit` https://github.com/haiwen/seafile-client/blob/master/src/seafile-applet.cpp#L263

Maybe we should do these things there?