Screen cleanups and refactoring

Work towards separating out how Screen interacts with other stuff.
* `Screen` should now be thread-safe. All commands to it are put in a
  queue and handled in `doTask` from the `loop()` task.
* Break dependency from `BluetoothUtil` to `Screen` by changing the
  pairing request into a callback.
* All accesses to screen now happen through the class.
* Fix `drawRows` so that the text scrolls along with frame animations.
* Remove example code that wasn't used.

src/main.h

@@ -1,6 +1,11 @@
 #pragma once
 
+#include "screen.h"
+
 extern bool axp192_found;
 extern bool ssd1306_found;
 extern bool isCharging;
-extern bool isUSBPowered;
+extern bool isUSBPowered;
+
+// Global Screen singleton.
+extern meshtastic::Screen screen;