Simplify handling of modifier-only keys

Now that we don't rely on str(KeyInfo) being empty anywhere, there's no reason
to return an empty string for only-modifier keypresses anymore.

While those keys can't be bound (QKeySequence('Shift') == Qt.Key_unknown)
there's also no reason to explicitly ignore them.

qutebrowser/keyinput/basekeyparser.py

@@ -128,10 +128,6 @@ class BaseKeyParser(QObject):
         txt = str(keyutils.KeyInfo.from_event(e))
         self._debug_log("Got key: 0x{:x} / text: '{}'".format(key, txt))
 
-        if keyutils.is_modifier_key(key):
-            self._debug_log("Ignoring, only modifier")
-            return QKeySequence.NoMatch
-
         if (txt.isdigit() and self._supports_count and not
                 (not self._count and txt == '0')):
             assert len(txt) == 1, txt

qutebrowser/keyinput/keyutils.py

@@ -200,15 +200,14 @@ class KeyInfo:
         """Convert this KeyInfo to a meaningful name.
 
         Return:
-            A name of the key (combination) as a string or
+            A name of the key (combination) as a string.
-            an empty string if only modifiers are pressed.
         """
-        if is_modifier_key(self.key):
-            return ''
-
         key_string = _key_to_string(self.key)
 
-        if is_printable(self.key):
+        if is_modifier_key(self.key):
+            # Don't return e.g. <Shift+Shift>
+            return '<{}>'.format(key_string)
+        elif is_printable(self.key):
             # "normal" binding
             # FIXME Add a test to make sure Tab doesn't become TAB
             assert len(key_string) == 1 or self.key == Qt.Key_Space, key_string

tests/unit/keyinput/test_keyutils.py

@@ -113,13 +113,13 @@ class TestKeyEventToString:
         """Test keyeevent when only control is pressed."""
         evt = fake_keyevent_factory(key=Qt.Key_Control,
                                     modifiers=Qt.ControlModifier)
-        assert not str(keyutils.KeyInfo.from_event(evt))
+        assert str(keyutils.KeyInfo.from_event(evt)) == '<Control>'
 
     def test_only_hyper_l(self, fake_keyevent_factory):
         """Test keyeevent when only Hyper_L is pressed."""
         evt = fake_keyevent_factory(key=Qt.Key_Hyper_L,
                                     modifiers=Qt.MetaModifier)
-        assert not str(keyutils.KeyInfo.from_event(evt))
+        assert str(keyutils.KeyInfo.from_event(evt)) == '<Hyper L>'
 
     def test_only_key(self, fake_keyevent_factory):
         """Test with a simple key pressed."""