Anki add-on: allow narrower main window (reduce minimum width)

for details see https://anki.tenderapp.com/discussions/ankidesktop/34122-how-to-reduce-the-width-of-the-main-window-for-anki-21 The code of this add-on was made by the Anki creator. I uploaded this as an add-on to make it more discoverable and easier to use.