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.
It is not required to decorate this value here, since a bool is an atomic value. Readable and writable with one machine instruction. However the actual size is up to the compiler and the target platform. 
Some time ago I had a discussion about this with RJ and the conclusion was that it is a good Idea to use here a QAtomicInt, it improve the readability is atomic without a doubt is a volatile int and should not have any overhead due to its inline implementation.