Create a new QStyleOptionTab object for each tab.

It seems under some circumstances (on Gentoo?), the existing QStyleOptionTab
object was reused, causing subsequent tabs to have the same title as the first
one.

Fixes #778.

qutebrowser/mainwindow/tabwidget.py

@@ -391,9 +391,9 @@ class TabBar(QTabBar):
     def paintEvent(self, _e):
         """Override paintEvent to draw the tabs like we want to."""
         p = QStylePainter(self)
-        tab = QStyleOptionTab()
         selected = self.currentIndex()
         for idx in range(self.count()):
+            tab = QStyleOptionTab()
             self.initStyleOption(tab, idx)
             if idx == selected:
                 bg_color = config.get('colors', 'tabs.bg.selected')