Encapsulate common display code in a singleton (#12395)

* Encapsulate common LCD code in a singleton
* Depend more UBL code on UBL_DEVEL_DEBUGGING
  - Since most users don't need the debugging on at all times, this helps reduce the default build size for UBL by over 2K, a little closer to fitting on 128K boards.

Marlin/src/gcode/bedlevel/mbl/G29.cpp

@@ -90,7 +90,7 @@ void GcodeSuite::G29() {
     case MeshStart:
       mbl.reset();
       mbl_probe_index = 0;
-      if (!lcd_wait_for_move) {
+      if (!ui.wait_for_bl_move) {
         enqueue_and_echo_commands_P(PSTR("G28\nG29 S2"));
         return;
       }
@@ -151,7 +151,7 @@ void GcodeSuite::G29() {
         #endif
 
         #if ENABLED(LCD_BED_LEVELING)
-          lcd_wait_for_move = false;
+          ui.wait_for_bl_move = false;
         #endif
       }
       break;