Fix alignment of tracks on Cover panel (close #106)

GTK 4 centers marks of the Scale widget even when the scale has a vertical
orientation. Unfortunately, the Scale widget does not provide a way to set the
alignment or to access the internal Label widget in any way. To left-align the
labels this commit add a method that traverses the all children of the songs
scale recursively and adjusts the alignment if it is a Label widget.

src/coverpanel.py

@@ -180,6 +180,21 @@ class CoverPanel(Gtk.Overlay):
             length, Gtk.PositionType.RIGHT,
             "{0[0]:02d}:{0[1]:02d} minutes".format(divmod(length, 60)))
 
+        # Align marks
+        self._align_songs_scale_marks()
+
+    def _align_songs_scale_marks(self):
+        self._align_songs_scale_mark(self.songs_scale)
+
+    def _align_songs_scale_mark(self, widget):
+        child = widget.get_first_child()
+        while child:
+            if type(child) is Gtk.Label:
+                child.set_halign(Gtk.Align.START)
+            else:
+                self._align_songs_scale_mark(child)
+            child = child.get_next_sibling()
+
     def _enable_tracklist(self):
         if self._current_album:
             # enable