Working on the Screen code

[davepl]

Initially, the code was written to display on the Heltec OLED. Then along came the LCD boards, the TFT boards, and M5, and so on. The code started to get pretty kludgy with ifdefs and so I decided to refactor it.

Now there's a Screen class that you talk to and hardware-specific versions override virtual functions like "writePixel" in the base. They can also override fillRect and drawLine and so on if there are hardware-accelerated versions of them.

One benefit/cost is that the text rendering now uses the AdaFruit_GFX interface. On the plus side the text looks the same on all devices, but it's not as fast as some of the native text routines. To keep things sized the same everywhere, I'm trying to avoid using any of the device-native text versions.

Here's a pic of the TFT S3 Feather board and the M5StickCPlus. Those are the two I've completed.... now OLED and LCD to go!