Implement a post-push signal that can be requested by callers of `set_pixels`

[nyaray]

If SOME OPTION is set in calls to set_pixels, Inky could send a message to those that set it when a push to the display is finally made.

This might help with (among other things) rate limiting, for example, a Scenic driver.

[lawik]

Don't we have a basically synchronous call? So we can actually just make the driver renderer wait until Inky is good and ready before it does anything else?

[nyaray]

It would allow the driver to work while the display renders.

(I know this is kind of a BS feature, just wrote it down for fun, hehe)