big, unsettling refactor of gui.document et al.

- split the class into a doc + windowcontroller which can (eventually)
also be used for task.py’s nib
- got autosave/resume working without hardcoding paths in the defaults
plist
- broke task.py in the process (stay tuned)
- fixed the lingering no-content bug when the app creates a `default’
untitled document on-foregrounding with no active windows
- started using File Coordinators rather than fsevents to reload open
docs with external modifications (though it blows away any undo state
in the process, so beware…)
- went back to using icns files (since drawing in a dock tile didn’t
actually get rid of the awful fringing)
- abandoned the ValueLadder and AskString widgets
- fixed a ton of memory leaks from closed windows not breaking
reference cycles
- fixed crashes related to closed documents not resigning from
notifications