Correct keybindings to deprecated commands.

Needed for #564 (because of :search "").
Also see #525.

qutebrowser/config/configdata.py

@@ -1179,3 +1179,14 @@ KEY_DATA = collections.OrderedDict([
         ('rl-backward-delete-char', ['<Ctrl-H>']),
     ])),
 ])
+
+
+# A dict of {old_cmd: new_cmd} strings.
+
+CHANGED_KEY_COMMNADS = {
+    'open -t about:blank': 'open -t',
+    'open -b about:blank': 'open -b',
+    'open -w about:blank': 'open -w',
+    'download-page': 'download',
+    'cancel-download': 'download-cancel',
+}

qutebrowser/config/parsers/keyconf.py

@@ -255,7 +255,7 @@ class KeyConfigParser(QObject):
             command = line.split(maxsplit=1)[0]
             if command not in cmdutils.cmd_dict:
                 raise KeyConfigError("Invalid command '{}'!".format(command))
-            self._cur_command = line
+            self._cur_command = configdata.CHANGED_KEY_COMMNADS.get(line, line)
 
     def _read_keybinding(self, line):
         """Read a key binding from a line."""