Rework symbol chooser code

The symbol chooser is now owned by DocumentView instead of DocumentController.

The symbol chooser does not know about its owner and is not an observer of any notifications. Instead it has a property for “document” and one for “selectionString”. It is the owners job to update these properties (i.e. when switching document or changing the selection).
Fix crash when pasting in file browser

This started after upgrading to clang-425.0.24. More info at https://github.com/sorbits/rdar/tree/master/for-in-with-ARC