Write documentation for sugar3.graphics.ColorToolButton

- add one example,

- part of feature Sugar3 Docs,
  https://wiki.sugarlabs.org/go/Features/Sugar3_Docs

Added documentation and example for Colorbutton

examples/colorbutton.py

@@ -0,0 +1,35 @@
+from gi.repository import Gtk
+
+from sugar3.graphics.toolbarbox import ToolbarBox
+from sugar3.graphics.colorbutton import ColorToolButton
+
+
+import common
+
+
+test = common.Test()
+test.show()
+
+vbox = Gtk.VBox()
+test.pack_start(vbox, True, True, 0)
+vbox.show()
+
+toolbar_box = ToolbarBox()
+vbox.pack_start(toolbar_box, False, False, 0)
+toolbar_box.show()
+
+separator = Gtk.SeparatorToolItem()
+toolbar_box.toolbar.insert(separator, -1)
+separator.show()
+
+def color_changed_cb(button, pspec):
+    print button.get_color()
+
+color_button = ColorToolButton()
+color_button.connect("notify::color", color_changed_cb)
+toolbar_box.toolbar.insert(color_button, -1)
+color_button.show()
+
+
+if __name__ == '__main__':
+    common.main(test)