Allow missing fixed_keys for configtypes.Dict

We just fill them up with a None value for the value type, so we can e.g. only
specify a subset of modes for bindings and the rest is {}.

qutebrowser/config/configdata.yml

@@ -2095,6 +2095,7 @@ bindings.commands:
                  'prompt', 'caret', 'register']
     valtype:
       name: Dict
+      none_ok: true
       keytype: Key
       valtype:
         name: Command

qutebrowser/config/configtypes.py

@@ -1031,10 +1031,11 @@ class Dict(BaseType):
         self.required_keys = required_keys
 
     def _validate_keys(self, value):
-        if (self.fixed_keys is not None and
-                value.keys() != set(self.fixed_keys)):
+        if (self.fixed_keys is not None and not
+                set(value.keys()).issubset(self.fixed_keys)):
             raise configexc.ValidationError(
                 value, "Expected keys {}".format(self.fixed_keys))
+
         if (self.required_keys is not None and not
                 set(self.required_keys).issubset(value.keys())):
             raise configexc.ValidationError(
@@ -1055,19 +1056,25 @@ class Dict(BaseType):
         self.to_py(yaml_val)
         return yaml_val
 
+    def _fill_fixed_keys(self, value):
+        """Fill missing fixed keys with a None-value."""
+        if self.fixed_keys is None:
+            return value
+        for key in self.fixed_keys:
+            if key not in value:
+                value[key] = self.valtype.to_py(None)
+        return value
+
     def to_py(self, value):
         self._basic_py_validation(value, dict)
         if not value:
-            if self.fixed_keys is None:
-                return {}
-            else:
-                return {key: self.valtype.to_py(None)
-                        for key in self.fixed_keys}
+            return self._fill_fixed_keys({})
 
         self._validate_keys(value)
 
-        return {self.keytype.to_py(key): self.valtype.to_py(val)
-                for key, val in value.items()}
+        d = {self.keytype.to_py(key): self.valtype.to_py(val)
+             for key, val in value.items()}
+        return self._fill_fixed_keys(d)
 
     def to_str(self, value):
         if not value:

tests/unit/config/test_configtypes.py

@@ -1398,7 +1398,7 @@ class TestDict:
         assert typ.from_str('{"answer": 42}') == {"answer": 42}
 
     @pytest.mark.parametrize('kind, val, ok', [
-        ('fixed', {"one": "1"}, False),  # missing key
+        ('fixed', {"one": "1"}, True),  # missing key (gets filled with None)
         ('fixed', {"one": "1", "two": "2", "three": "3"}, False),  # extra key
         ('fixed', {"one": "1", "two": "2"}, True),
 
@@ -1410,7 +1410,7 @@ class TestDict:
     def test_keys(self, klass, kind, val, ok, from_str):
         if kind == 'fixed':
             d = klass(keytype=configtypes.String(),
-                      valtype=configtypes.String(),
+                      valtype=configtypes.String(none_ok=True),
                       fixed_keys=['one', 'two'])
             message = 'Expected keys .*'
         elif kind == 'required':