Improve performance of startup and shutdown

We call 'update_tab_titles' a lot of times which calls 'setTabText' on
every tab. 'setTabText' calls tabSizeHint and minTabSizeHint on every
tab as well, meaning this is an n^2 operation repeated many times.

First, this prevents setTabText from being called unless it's needed,
removing most of the work done.

Second, I remove tabs in reverse, to avoid recomputing the above for
every tab on shutdown (which is at least n^3)

qutebrowser/mainwindow/tabbedbrowser.py

@@ -253,7 +253,10 @@ class TabbedBrowser(tabwidget.TabWidget):
     def shutdown(self):
         """Try to shut down all tabs cleanly."""
         self.shutting_down = True
-        for tab in self.widgets():
+        # Reverse tabs so we don't have to recacluate tab titles over and over
+        # Removing first causes [2..-1] to be recomputed
+        # Removing the last causes nothing to be recomputed
+        for tab in reversed(self.widgets()):
             self._remove_tab(tab)
 
     def tab_close_prompt_if_pinned(

qutebrowser/mainwindow/tabwidget.py

@@ -150,8 +150,9 @@ class TabWidget(QTabWidget):
         title = '' if fmt is None else fmt.format(**fields)
         tabbar = self.tabBar()
 
-        tabbar.setTabText(idx, title)
-        tabbar.setTabToolTip(idx, title)
+        if tabbar.tabText(idx) != title:
+            tabbar.setTabText(idx, title)
+            tabbar.setTabToolTip(idx, title)
 
     def get_tab_fields(self, idx):
         """Get the tab field data."""