ObjectListModel: Fix assert in objectRemovedMainThread

It's not safe to assume that m_invalidatedObjects does not contain obj in
case objectRemovedMainThread is called from the main thread.

We can end up in this order of calls:
- objectRemoved from a thread other than the main thread
  - inserts obj into m_invalidatedObjects
  - queues a call to objectRemovedMainThread
- objectAdded with the same obj-pointer
- objectRemoved from the main thread
  - calls objectRemovedMainThread directly
  - m_invalidatedObjects still contains obj.
=> Assertion triggered

We fix this by always trying to remove obj from m_invalidatedObjects in
objectRemovedMainThread.
Merge pull request #73 from krf/fix-multithreaded

ObjectListModel: Fix assert in objectRemovedMainThread