PaletteWindowWidget: set border width to make outer space in the container - SL #4295

In the palette reimplamentation, commit 48ad255a, set_border_width was
removed.  Add it again to the corresponding widget, and get the value
from the theme.  Previously the value was given by:
self.get_style().xthickness .  Now we can use the widget border.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/graphics/palettewindow.py

@@ -278,6 +278,11 @@ class _PaletteWindowWidget(Gtk.Window):
         self.set_resizable(False)
         self.set_position(Gtk.WindowPosition.NONE)
 
+        context = self.get_style_context()
+        # Just assume all borders are the same
+        border = context.get_border(Gtk.StateFlags.ACTIVE).right
+        self.set_border_width(border)
+
         accel_group = Gtk.AccelGroup()
         self.sugar_accel_group = accel_group
         self.add_accel_group(accel_group)