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/MeshService.cpp

@@ -2,12 +2,12 @@
 #include <Arduino.h>
 #include <assert.h>
 
+#include "main.h"
 #include "mesh-pb-constants.h"
 #include "MeshService.h"
 #include "MeshBluetoothService.h"
 #include "NodeDB.h"
 #include "GPS.h"
-#include "screen.h"
 #include "Periodic.h"
 #include "PowerFSM.h"
 
@@ -118,7 +118,7 @@ MeshPacket *MeshService::handleFromRadioUser(MeshPacket *mp)
         sendOurOwner(mp->from);
 
         String lcd = String("Joined: ") + mp->payload.variant.user.long_name + "\n";
-        screen_print(lcd.c_str());
+        screen.print(lcd.c_str());
     }
 
     return mp;
@@ -360,4 +360,4 @@ void MeshService::onNotify(Observable *o)
 {
     DEBUG_MSG("got gps notify\n");
     onGPSChanged();
-}
+}