isn't closed

the fact that last-bound handlers are called first, and there is no event you
can bind to which says that a frame is actually closing - close frame
handlers (currently just the shutdown function) must be managed by the
FSLeyesFrame.

must be called before application exits, to clear the GL context and
dummy canvas/frame.

parent argument - GLContext always creates its own dummy frame

raises

a "jupyter-notebook" command - nbmain wrapper function is always used.

command line

creates a wx.App if necessary.

can't set taskbar icon.

accept two parameters - a list of paths, and a list of overlays. Also added
rudimentary support for complex images - a complex image is split into two
overlays (one with the real part, and one with the imaginary part). Hence
the need for the refactoring.

dummy modaldialoghook if wxplatform is unknown

directly (via FSLeyesFrame.menuActions)

application.

fsleyes.initialise()

can control the command-line call to ipykernel_launcher.

workaround than using os.fork or multiprocessing.

things quiet.

signal handler. Notebook server tries a bit harder to make sure that the
server process is killed. Still not confident as to the robustness of all of
this.