Theme: improve notification scrollbar appearance

Message tray: modify bubble scrollview notification padding to
have the same padding at right, bottom, top and left since
having different padding make scrollbar looks missaligned.
Also adjust border radius of bubble to not clash the scrollbar.

https://bugzilla.gnome.org/show_bug.cgi?id=688393

data/theme/gnome-shell.css

@@ -1310,7 +1310,7 @@ StScrollBar StButton#vhandle:active {
 }
 
 .summary-boxpointer {
-    -arrow-border-radius: 8px;
+    -arrow-border-radius: 15px;
     -arrow-background-color: rgba(0,0,0,0.8);
     -arrow-base: 36px;
     -arrow-rise: 18px;
@@ -1331,8 +1331,8 @@ StScrollBar StButton#vhandle:active {
 
 .summary-notification-stack-scrollview {
     max-height: 18em;
-    padding-top: 6px;
-    padding-bottom: 6px;
+    padding-top: 8px;
+    padding-bottom: 8px;
 }
 
 .summary-notification-stack-scrollview:ltr {