Add an lru cache for WebHistoryInterface.historyContains

When loading heise.de, for some crazy reason QtWebKit calls historyContains
about 16'000 times.

With this cache (which we simply clear when *any* page has been loaded, as then
the links which have been visited can change), that's down to 250 or so...

qutebrowser/browser/webkit/webkithistory.py

@@ -19,6 +19,7 @@
 
 """QtWebKit specific part of history."""
 
+import functools
 
 from PyQt5.QtWebKit import QWebHistoryInterface
 
@@ -36,11 +37,13 @@ class WebHistoryInterface(QWebHistoryInterface):
     def __init__(self, webhistory, parent=None):
         super().__init__(parent)
         self._history = webhistory
+        self._history.changed.connect(self.historyContains.cache_clear)
 
     def addHistoryEntry(self, url_string):
         """Required for a QWebHistoryInterface impl, obsoleted by add_url."""
         pass
 
+    @functools.lru_cache(maxsize=32768)
     def historyContains(self, url_string):
         """Called by WebKit to determine if a URL is contained in the history.
 

qutebrowser/misc/utilcmds.py

@@ -170,8 +170,15 @@ def debug_cache_stats():
     """Print LRU cache stats."""
     config_info = objreg.get('config').get.cache_info()
     style_info = style.get_stylesheet.cache_info()
+    try:
+        from PyQt5.QtWebKit import QWebHistoryInterface
+        interface = QWebHistoryInterface.defaultInterface()
+        history_info = interface.historyContains.cache_info()
+    except ImportError:
+        history_info = None
     log.misc.debug('config: {}'.format(config_info))
     log.misc.debug('style: {}'.format(style_info))
+    log.misc.debug('history: {}'.format(history_info))
 
 
 @cmdutils.register(debug=True)