Provide utilities to gracefully shut down the GTK event loop about owlkettle

[can-lehmann]

Sorry for the delay, I am currently recovering from COVID. According to https://discourse.gnome.org/t/best-practice-to-quit-a-gtkapplication/3660, you just need to close all windows. Something like this might work:

import owlkettle

viewable App:
  discard

method view(app: AppState): Widget =
  result = gui:
    Window:
      Box:
        margin = 12
        Button:
          text = "Quit"
          
          proc clicked() =
            proc closeTask(): bool =
              app.closeWindow()
            
            discard addGlobalIdleTask(closeTask)

brew(gui(App()))

I call close window inside an idleTask here to prevent a potential use after free, since clicked will trigger a redraw after it is called.

Edit: You might need to deal with open dialogs separately.

Edit: Added scheduleCloseWindow to prevent use after free. ab5cb84