Remove race possibilities from slip mode

I think this was causing https://bugs.launchpad.net/mixxx/+bug/1362394, and I know for sure there were races in this code, so even if it doesn't fix that bug it's an improvement.
Yes, but only m_bSlipEnabled.  It sound crazy, but this a hint for later contributors that this is used by two threads in a atomic way. You can find similar here: 

https://github.com/mixxxdj/mixxx/blob/38dcd763c2ffc46e6958ca687a8a83e928059c4d/src/cachingreaderworker.h#L118
https://github.com/mixxxdj/mixxx/blob/45937ba3bde0ef86b740be9c504d675330b4c422/src/engine/enginebuffer.h#L359
https://github.com/mixxxdj/mixxx/blob/38dcd763c2ffc46e6958ca687a8a83e928059c4d/src/controller/hid/hidcontroller.h#L37
https://github.com/mixxxdj/mixxx/blob/38dcd763c2ffc46e6958ca687a8a83e928059c4d/src/util/statsmanager.h#L63

I have just noticed: 
Unfortunately RJ has introduced a dref() macro for Qt 5.0 compatible,  which really misleading since it is not QAtomicInt::def(). I think we should rename it to a_load() or something.  
Is not required in 5.3.  
Can you take care of it?