Improve thread safety in UI code

- Implemented better C++ classes for handling scenes/sources/items in
  obs.hpp, allowing them to automatically increment and decrement the
  references of each, as well as assign them to QVariants.

- Because QVariants are now using the C++ classes, remove the pointer
  QVariant wrapper.

- Use the new C++ classes with the QVariant user data of list box items,
  both for the sake of thread safety and to ensure that the data
  referenced is not freed until removed.  NOTE: still might need some
  testing.

- Implemented a source-remove signal from libobs, and start using that
  signal instead of the source-destroy signal for signalling item
  removal.