Tray: set a minimum size in the viewport so that it allows scrolling

The minimum height/width requested by the GtkViewport still tries to cater
for all contained children, which makes the [VH]Tray widgets to grow as
new items are added. Instead, request a minimum width/height of 0 to avoid
the Tray from growing, and having scrolling kick in instead.

Also, fixed what seemed to be a typo in do_get_preferred_height(), where
the viewport width was requested instead.

http://bugs.sugarlabs.org/ticket/3522

Signed-off-by: Carlos Garnacho <carlos@lanedo.com>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/graphics/tray.py

@@ -115,13 +115,15 @@ class _TrayViewport(Gtk.Viewport):
 
     def do_get_preferred_width(self):
         if self.orientation == Gtk.Orientation.HORIZONTAL:
-            return Gtk.Viewport.do_get_preferred_width(self)
+            min_width, nat_width = Gtk.Viewport.do_get_preferred_width(self)
+            return 0, nat_width
         child_minimum, child_natural = self.get_child().get_preferred_size()
         return child_minimum.width, child_natural.width
 
     def do_get_preferred_height(self):
         if self.orientation != Gtk.Orientation.HORIZONTAL:
-            return Gtk.Viewport.do_get_preferred_width(self)
+            min_height, nat_height = Gtk.Viewport.do_get_preferred_height(self)
+            return 0, nat_height
         child_minimum, child_natural = self.get_child().get_preferred_size()
         return child_minimum.height, child_natural.height