Specify completion category font in HTML

It seems like specifying a font for QTreeView::item in a Qt stylesheet
doesn't actually do anything.

qutebrowser/completion/completiondelegate.py

@@ -206,7 +206,10 @@ class CompletionItemDelegate(QStyledItemDelegate):
             else:
                 self._doc.setPlainText(self._opt.text)
         else:
-            self._doc.setHtml(html.escape(self._opt.text))
+            self._doc.setHtml(
+                '<span style="font: {};">{}</span>'.format(
+                    html.escape(config.get('fonts', 'completion.category')),
+                    html.escape(self._opt.text)))
 
     def _draw_focus_rect(self):
         """Draw the focus rectangle of an ItemViewItem."""

qutebrowser/completion/completionwidget.py

@@ -69,7 +69,6 @@ class CompletionView(QTreeView):
                 {{ color['completion.category.border.top'] }};
             border-bottom: 1px solid
                 {{ color['completion.category.border.bottom'] }};
-            font: {{ font['completion.category'] }};
         }
 
         QTreeView::item:selected, QTreeView::item:selected:hover {