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/PowerFSM.cpp

@@ -179,4 +179,4 @@ void PowerFSM_setup()
     // timeout");
 
     powerFSM.run_machine(); // run one interation of the state machine, so we run our on enter tasks for the initial DARK state
-}
+}