* ARCify VT100WorkingDirectory, iTermImageInfo, iTermMark.
* Move code->iTermImageInfo map into a new class, ImageRegistery.
* Make various classes immutable (VT100RemoteHost, VT100WorkingDirectory, iTermCapturedOutputMark, iTermImageMark)
* Make LineBlock's observers an ObjC array because it's impossible to reason about object lifetimes in C++ data structures.
* Fix a leak of metadata line copied line blocks.
* Fix use after dealloc of _shell when replacing a terminated session.
* Fix leak of VT100ScreenState in sync.
* Don't register VT100ScreenMark doppelgangers.
* Fix oob write when changing cusor type while it's in the overflow column
* Fix a race that made restoring images not always work (garbage collected prematurely)