Fix crash when clicking outline (#1299)

On PyQt5 5.15.11 (the current latest), subElementRect (from qproxystyle)
expects three arguments, not two. This isn't consistent with the other
subElementRect functions, so I strongly suspect that the problem is
somewhere upstream.

I see this behavior in the docs and in the package itself, so I don't
see a very good way around it. The bindings for subElementRect aren't
consistent:
```
bindings/QtWidgets/qcommonstyle.sip
39:    virtual QRect subElementRect(QStyle::SubElement r, const QStyleOption *opt, const QWidget *widget = 0) const;

bindings/QtWidgets/qproxystyle.sip
41:    virtual QRect subElementRect(QStyle::SubElement element, const QStyleOption *option, const QWidget *widget) const;

bindings/QtWidgets/qstyle.sip
270:    virtual QRect subElementRect(QStyle::SubElement subElement, const QStyleOption *option, const QWidget *widget = 0) const = 0;
```

(`widget` in `qproxystyle` should have a default value of `0`/`nullptr`,
but it doesn't)

This also shows up in the docs. These have a default:
https://www.riverbankcomputing.com/static/Docs/PyQt5/api/qtwidgets/qcommonstyle.html##subElementRect
https://www.riverbankcomputing.com/static/Docs/PyQt5/api/qtwidgets/qstyle.html##subElementRect
but the one we're using here doesn't:
https://www.riverbankcomputing.com/static/Docs/PyQt5/api/qtwidgets/qproxystyle.html##subElementRect

Even if other versions have a default value, it should work fine as a
positional argument, and `None` would be the default value anyway.

manuskript/ui/views/outlineDelegates.py

@@ -29,8 +29,8 @@ def paint(self, painter, option, index):
         opt = QStyleOptionViewItem(option)
         self.initStyleOption(opt, index)
 
-        iconRect = style.subElementRect(style.SE_ItemViewItemDecoration, opt)
-        textRect = style.subElementRect(style.SE_ItemViewItemText, opt)
+        iconRect = style.subElementRect(style.SE_ItemViewItemDecoration, opt, None)
+        textRect = style.subElementRect(style.SE_ItemViewItemText, opt, None)
 
         # Background
         style.drawPrimitive(style.PE_PanelItemViewItem, opt, painter)