Fixing leaks in Mixxx

Most fixes are just freeing resources. std::vector does need more tricks to be surely it should be done this way: http://www.cplusplus.com/reference/vector/vector/clear/ (swap) not like it's done now with resize(0) (it works 90% times but not allways). Also share_ptr stuff make sometimes my hair fall.. Every commit have valgrind memory leak it's related.
EngineDeck outlives the BaseTrackPlayer so this is unsafe