Lowercase special keys when checking if they're new

Between v0.8.4 and now, the default keybinding for Ctrl-PgDown changed:

    - ('tab-focus', ['J', '<Ctrl-PgDown>']),
    + ('tab-next', ['J', '<Ctrl-PgDown>']),

The existing keybinding was lower-cased in the config, but _is_new
didn't lowercase the new one, causing a conflict to show up.

Fixes #1835
See #1958
Supersedes #1986

qutebrowser/config/parsers/keyconf.py

@@ -280,6 +280,9 @@ class KeyConfigParser(QObject):
         A binding is considered new if both the command is not bound to any key
         yet, and the key isn't used anywhere else in the same section.
         """
+        if utils.is_special_key(keychain):
+            keychain = keychain.lower()
+
         try:
             bindings = self.keybindings[sectname]
         except KeyError: