More memory leaks (#215)

* Clean up leaks from MapView enter/exit events

* clean up a lingering toolbar button

* Fix a leak in AudioMultiPooledFX.PooledAudioSource

* Fix leaking landed and splashed vessels on kerbin via KSCVesselMarkers

* Fix #203: clean up leaked clone of root part prefab

* clear the FlightGlobals part caches on scene unload

* when parts are destroyed, unhook references from the ProtoPartSnapshot

* when vessels are unloaded, clean up their PartSets and some part references

* nullcheck for partset

* more nullchecks for partSet

* Free up references when disconnecting autostruts

* clear Part.allparts on scene unload

* Fix some issues that could leak parts and vessels via ProtoCrewMember

* make the TimingManager cleanup more generalized so we can report leaks from mods (there was one in KOS)

* More cleanup when a vessel is unloaded