Bind back/forward keys by default

Those are the XF86Back/Forward media keys on a keyboard, which are apparently
handled by default with QtWebKit, but not QtWebEngine.

Fixes #2166.

qutebrowser/config/configdata.py

@@ -1562,10 +1562,10 @@ KEY_DATA = collections.OrderedDict([
         ('tab-clone', ['gC']),
         ('reload', ['r', '<F5>']),
         ('reload -f', ['R', '<Ctrl-F5>']),
-        ('back', ['H']),
+        ('back', ['H', '<back>']),
         ('back -t', ['th']),
         ('back -w', ['wh']),
-        ('forward', ['L']),
+        ('forward', ['L', '<forward>']),
         ('forward -t', ['tl']),
         ('forward -w', ['wl']),
         ('fullscreen', ['<F11>']),