Implement save_gif function

[martinRenou]

This PR makes saving frames more reliable (fix https://github.com/martinRenou/ipycanvas/issues/222), allowing to implement a save_gif method (fix https://github.com/martinRenou/ipycanvas/issues/136)

from time import sleep

# 50Hz
frequency = 20

with save_gif(canvas, "test.gif", frequency=frequency):
    for i in range(50):
        canvas.clear()

        draw(canvas, i * frequency)

        canvas.frame()

TODO

• Implement a save_video using OpenCV (as an optional dependency)
• Add docs for this
• Update the docs for saving files: using frame instead of sync_image_data
• Remove docs for sync_image_data and deprecate it? We should probably encourage manual syncing from now on
• Update self.data when a frame is received