Remove `background_color` and `clear_frame`

[Liamolucko]

trafficstars

Resolves #100

Removes background_color, set_background_color, and clear_frame, and properly handles the filling of the chromatic layer when rendering in black and white.

Looking back at #100, I'm not entirely clear if you wanted to keep or remove clear_frame; I'm happy to put it back taking the color to clear with as an argument.

I did find one display which was using background_color for stuff other than clear_frame - the epd5in16f was using it to set the border color. For now, I've replaced it with a set_border_color method which is only on that display. It should eventually be replaced by a proper method on WaveshareDisplay, but I don't have any hardware which supports border colors that I can test with.

[caemor]

Sorry for the late response I just realized that I never had published my review and it was still pending :see_no_evil:

[peckpeck]

I think we should just keep the clear_frame method, it is useful for initializing and testing the epd, especially when compiled without the graphic feature