New zoom function: `self.camera_frame` zooms at all visibile Mobjects

[kolibril13]

Currently, it is possible to set the width of the frame like this:

class Example1(MovingCameraScene):
    def construct(self):
        text = Text("Hello World")
        self.add(text)
        self.wait()
        self.camera_frame.set_width(text.get_width()*1.2)
        self.wait()

72c0d397490c7ced21478727e34edf6d3c251484.mp4

I think it would be nice to give self.camera_frame a further function, that scans the boundary points of all visible Mobjects and then mooves the current camera frame to that area.