UI: Fix ugly alignment of timeline popover buttons

Apparently things have changed since {rB98d205508977a6f72bf55},
and putting the buttons in two separate rows is no longer enough
to keep the right-most popover button in the timeline header from
aligning with the menu buttons. This just adds a separator large
enough to avoid the alignment.

release/scripts/startup/bl_ui/space_time.py

@@ -107,6 +107,9 @@ class TIME_MT_editor_menus(Menu):
             text="Keying",
         )
 
+        # Add a separator to keep the popover button from aligning with the menu button.
+        sub.separator(factor=0.4)
+
         if horizontal:
             sub = row.row(align=True)