Gridbox enhancement

[hainm]

• add handle_resize for backend and handleResszie for frontend: The idea is that we can properly resize the view's container if the view is in nested boxes.
• add EXPERIMENTAL Fullscreen class to handle fullscreen.
• Add syncing repr between two views of two models (I am not sure about this usage).

[hainm]

before:

after:

PS: ideally we don't need handle_resize at all. Don't have clean implementation yet.

[hainm]

may be

class GridBox2(Gridbox):
     def __init__(self, *args, **kwargs):
        [bla bla]
        self.on_displayed(self.handle_display)
    def handle_display(self):
        for v in self.children:
           v.handle_resize()