Add stubs for on_mode_{entered,left} for WebEngine

This means at least we won't get a crash when using the commandline.

qutebrowser/browser/webengine/webenginetab.py

@@ -45,6 +45,16 @@ class WebEngineCaret(tab.AbstractCaret):
 
     ## TODO
 
+    @pyqtSlot(usertypes.KeyMode)
+    def on_mode_entered(self, mode):
+        ## TODO
+        pass
+
+    @pyqtSlot(usertypes.KeyMode)
+    def on_mode_left(self, mode):
+        ## TODO
+        pass
+
     def has_selection(self):
         return self._widget.hasSelection()