v0.45.1: the laser's Afterimage is a number box, like Line width

Type the seconds, or step by 0.05 with the arrows or ↑/↓; it reads "Off"
at zero. Line width and Afterimage now share one number-box helper.

tests/test_round79.py

@@ -282,3 +282,16 @@ class TestCopyAsText:
         from lintunes.gui.track_table import song_line
         from lintunes.models import Track
         assert song_line(Track(track_id=9, name="", artist="X")) == "X - Untitled"
+
+
+class TestAfterimageBox:
+    def test_steps_by_a_twentieth_and_reads_off_at_zero(self, qapp, tmp_path):
+        prefs = Preferences(tmp_path)
+        box = _prefs_dialog(prefs)._laser_fade_box
+        assert box.value() == pytest.approx(0.1)       # the default
+        box.stepUp()
+        assert prefs.get("visualizer_laser_afterimage") == pytest.approx(0.15)
+        box.setValue(0)
+        assert box.text() == "Off"
+        box.setValue(99)                                # past the top
+        assert box.value() == 3.0