Add top and bottom shadows to app browser

Add a 'vshadow' property to StScrollView, which, when turned on,
overlays gradient shadows on the top and bottom of the StScrollView.

Turn this on for the StScrollView used for the app browser.
https://bugzilla.gnome.org/show_bug.cgi?id=609604

src/st/st-scroll-view.h

@@ -88,6 +88,9 @@ void          st_scroll_view_set_policy          (StScrollView   *scroll,
                                                   GtkPolicyType   hscroll,
                                                   GtkPolicyType   vscroll);
 
+void          st_scroll_view_set_vshadows        (StScrollView *self,
+                                                  gboolean      vshadows);
+
 G_END_DECLS
 
 #endif /* __ST_SCROLL_VIEW_H__ */