Instances of the Editor class are leaked.

[superfell]

Describe the bug
When a main soqlx window is closed, the associated Explorer class instance is not deallocated.

Discovered during work on #138 which had similar issues for some of its UI related classes that load UIs from Nibs.

As described in ZKLoginController, any UI component that uses bindings, and binds to the File Owner will create a retain cycle which will prevent the class from being deallocated.

The fix is the same as in ZKLoginController, move the state that the UI binds to into a separate class/instance from the File Owner instance. There's quite a bit of binding in the main UI window so this is a larger fix than the login ones.