Add new PaletteMenuItem widget for usage in mixed content Palettes

As Palettes can either include a Gtk.Menu or a Gtk.Window we
have to use a "false" menu when we want to have both
functionality in a Palette. This is a new class PaletteMenuItem
for those use cases.

Code highly based on the work from Gonzalo Odiard. The API is
based on the one from GtkImageMenuItem [1].

[1] http://developer.gnome.org/gtk3/3.4/GtkImageMenuItem.html

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

src/sugar3/graphics/Makefile.am

@@ -12,6 +12,7 @@ sugar_PYTHON =              \
 	objectchooser.py        \
 	palettegroup.py         \
 	palette.py              \
+	palettemenuitem.py	\
 	palettewindow.py        \
 	panel.py                \
 	radiopalette.py         \