Update for Xcode 6 beta 6.

Behavioral changes:
- QRevisionTracker now has a group() method for grouping undo/redo
  operations.
- A few assertions were removed.
- Some use of optional assignment (i.e., `obj?.property = value`). More
  use of this could be made.
- Most errors fixed had to do with checking for whether a property is
  defined or undefined (i.e., replacing `x?` and `!x` with `x != nil`
  and `x == nil` because Apple came up with a sensible way of doing
  this).
- Uses the null-coalescing operator where it makes sense. This actually
  is a kind of set-back because there's no null-coalescing assignment
  operator (e.g., ??=) so there's at least one case of `x = x ?? y`
  and otherwise the semantics for that operator are still kind of weird
  because it's not always clear what's wrong if there's a compiler
  error around one. In fact, compiler errors seem to generally be worse
  in beta 6, so who knows. The type system seems to be a little over
  active with trying to pin an error on certain types.

Have not yet checked if the missing type issue for NSControl is
resolved.
Update SnowKit subrepo.