Fix MainWindow inheritance

[ferdnyc]

A change I left out of #3684 — MainWindow (and really all Qt-based classes) needs to have its Qt parent listed last in its parent class list, so that it will be the first lookup for all inherited members.

IOW,

class MainWindow(QMainWindow, UpdateWatcher):

Doesn't guarantee that super() calls will always go to the corresponding QMainWindow class members (if they're present). But,

class MainWindow(UpdateWatcher, QMainWindow):

does.

This especially matters for things like __init__ that lots of classes share, but AIUI it's also a good idea in general. Python's multiple-inheritance implementation received two massive overhauls back in the Python 3.2 - 3.4 dev cycles, before reaching its current form, and it can still be a bit shaky at times apparently.