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.
The problem was we were pulsing the display power briefly down while
reentering the ON state (because the ON states exit rule turned it off).
Instead we now just turn off the screen on entry to DARK or LS states
* Have state machine properly enter deep sleep based on loss of mesh and phone comms.
* Default to enter deep sleep if no LORA received for two hours (indicates user has probably left the mesh).
Signed-off-by: Kevin Hester <kevinh@geeksville.com>
