add gallery Previous,Next,Close,EnablePreview actions

I tryed to resolve the part of #22
I added below actions

        def Close(self):
            """
            :doc: gallery method
            
            Close the current image.
            """

        def Next(self, diff=False):
            """
            :doc: gallery method
            
            This is the action to advance to the next unlocked image.
            When the last image is showing, close it.

            `diff`
                If True, advance to the next image in the button, otherwise
                advance to the image in the next button.
            """

        def Previous(self):
            """
            :doc: gallery method
            
            This is the action to return to the previous unlocked image.
            When the first image is showing, close it.
            """

        def EnablePreview(self, preview_time=10):
            """
            :doc: gallery method
            
            This is the action to enable preview_mode.
            
            `preview_time`
                This is a number in second, defaults to 10. advance to the next unlocked image
                when this time is elapsed if preview_mode is enable.
            """

And I use "gallery" screen in place of "_gallery" if exist.

By the way, what is the cradle song image gallery meaning?