Notifications: add some top margin

68a5fcdb moved them a bit to near the top than before,
and it can overlap with CRE rendering progress bar.
Also makes it nicer when multiple notications are stacked.

frontend/ui/widget/notification.lua

@@ -84,7 +84,10 @@ function Notification:init()
             -- have this VerticalGroup full width, to ensure centering
             width = Screen:getWidth(),
             -- push this frame at its y=self.num position
-            height = notif_height * (self.num - 1),
+            height = notif_height * (self.num - 1) + self.margin,
+                -- (let's add a leading self.margin to get the same distance
+                -- from top of screen to first notification top border as
+                -- between borders of next notifications)
         },
         self.frame,
     }