Theme: improve menu popup appearance

Make menu popups fully match the mockups by changing the stroke
colour, pointer and corner radius size. This has the benefit of
distinguishing the menu from the background and gives it a subtler
appearance that fits the shell's aesthetic.

https://bugzilla.gnome.org/show_bug.cgi?id=655627

data/theme/gnome-shell.css

@@ -93,12 +93,12 @@ StTooltip StLabel {
 /* PopupMenu */
 
 .popup-menu-boxpointer {
-    -arrow-border-radius: 9px;
+    -arrow-border-radius: 8px;
     -arrow-background-color: rgba(0,0,0,0.9);
     -arrow-border-width: 2px;
-    -arrow-border-color: #5f5f5f;
+    -arrow-border-color: #a5a5a5;
-    -arrow-base: 30px;
+    -arrow-base: 24px;
-    -arrow-rise: 15px;
+    -arrow-rise: 11px;
 }
 
 .popup-menu {