add Gallery get_fraction

I add Gallery Class get_fraction func.
This enables the rate of seen_images all_images about the given button to be shown on the screen like below.


                            vbox:
                                add g.make_button("test2", "image/thum/stest02.png")
                                text g.get_fraction("test2") xalign 1.0


![screenshot0001](https://f.cloud.github.com/assets/1556311/725357/28137198-e05f-11e2-8871-558825d6b42f.png)