Add minimal key tester script.

See #658.

scripts/keytester.py

@@ -0,0 +1,23 @@
+from PyQt5.QtWidgets import QApplication, QWidget, QLabel, QHBoxLayout
+
+from qutebrowser.utils import utils
+
+
+class KeyWidget(QWidget):
+
+    """Widget displaying key presses."""
+
+    def __init__(self, parent=None):
+        super().__init__(parent)
+        self._layout = QHBoxLayout(self)
+        self._label = QLabel(text="Waiting for keypress...")
+        self._layout.addWidget(self._label)
+
+    def keyPressEvent(self, e):
+        self._label.setText(utils.keyevent_to_string(e))
+
+
+app = QApplication([])
+w = KeyWidget()
+w.show()
+app.exec_()