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.
Hm, these deletes aren't safe since the engine can have active pointers to them. Deletion of EngineEffects, EngineEffectChains and EngineEffectRacks needs to be deferred until we receive a success message for removing it from the engine.