Add RESTORE_LEVELING_AFTER_G28 option

.travis.yml

   - opt_set TEMP_SENSOR_3 20
   - opt_set TEMP_SENSOR_4 999
   - opt_set TEMP_SENSOR_BED 1
-  - opt_enable AUTO_BED_LEVELING_UBL DEBUG_LEVELING_FEATURE G26_MESH_EDITING ENABLE_LEVELING_FADE_HEIGHT EEPROM_SETTINGS EEPROM_CHITCHAT G3D_PANEL SKEW_CORRECTION
+  - opt_enable AUTO_BED_LEVELING_UBL RESTORE_LEVELING_AFTER_G28 DEBUG_LEVELING_FEATURE G26_MESH_EDITING ENABLE_LEVELING_FADE_HEIGHT EEPROM_SETTINGS EEPROM_CHITCHAT G3D_PANEL SKEW_CORRECTION
   - opt_enable_adv CUSTOM_USER_MENUS I2C_POSITION_ENCODERS BABYSTEPPING BABYSTEP_XY LIN_ADVANCE NANODLP_Z_SYNC QUICK_HOME
   - build_marlin_pio ${TRAVIS_BUILD_DIR} ${TEST_PLATFORM}
   #
   #
   - use_example_configs delta/generic
   - opt_disable DISABLE_MIN_ENDSTOPS
-  - opt_enable AUTO_BED_LEVELING_UBL Z_PROBE_ALLEN_KEY EEPROM_SETTINGS EEPROM_CHITCHAT OLED_PANEL_TINYBOY2 MESH_EDIT_GFX_OVERLAY
+  - opt_enable AUTO_BED_LEVELING_UBL RESTORE_LEVELING_AFTER_G28 Z_PROBE_ALLEN_KEY EEPROM_SETTINGS EEPROM_CHITCHAT OLED_PANEL_TINYBOY2 MESH_EDIT_GFX_OVERLAY
   - build_marlin_pio ${TRAVIS_BUILD_DIR} ${TEST_PLATFORM}
   #
   # Delta Config (FLSUN AC because it's complex)

Marlin/Configuration.h

 //#define MESH_BED_LEVELING
 
 /**
+ * Normally G28 leaves leveling disabled on completion. Enable
+ * this option to have G28 restore the prior leveling state.
+ */
+//#define RESTORE_LEVELING_AFTER_G28
+
+/**
  * Enable detailed logging of G28, G29, M48, etc.
  * Turn on with the command 'M111 S32'.
  * NOTE: Requires a lot of PROGMEM!

Marlin/src/config/default/Configuration.h

 //#define MESH_BED_LEVELING
 
 /**
+ * Normally G28 leaves leveling disabled on completion. Enable
+ * this option to have G28 restore the prior leveling state.
+ */
+//#define RESTORE_LEVELING_AFTER_G28
+
+/**
  * Enable detailed logging of G28, G29, M48, etc.
  * Turn on with the command 'M111 S32'.
  * NOTE: Requires a lot of PROGMEM!

Marlin/src/gcode/calibrate/G28.cpp

 
   // Disable the leveling matrix before homing
   #if HAS_LEVELING
-    #if ENABLED(AUTO_BED_LEVELING_UBL)
-      const bool ubl_state_at_entry = planner.leveling_active;
+    #if ENABLED(RESTORE_LEVELING_AFTER_G28)
+      const bool leveling_state_at_entry = planner.leveling_active;
     #endif
     set_bed_leveling_enabled(false);
   #endif
     do_blocking_move_to_z(delta_clip_start_height);
   #endif
 
-  #if ENABLED(AUTO_BED_LEVELING_UBL)
-    set_bed_leveling_enabled(ubl_state_at_entry);
+  #if ENABLED(RESTORE_LEVELING_AFTER_G28)
+    set_bed_leveling_enabled(leveling_state_at_entry);
   #endif
 
   clean_up_after_endstop_or_probe_move();

Marlin/src/inc/SanityCheck.h

     #error "AUTO_BED_LEVELING_UBL requires EEPROM_SETTINGS. Please update your configuration."
   #elif !WITHIN(GRID_MAX_POINTS_X, 3, 15) || !WITHIN(GRID_MAX_POINTS_Y, 3, 15)
     #error "GRID_MAX_POINTS_[XY] must be a whole number between 3 and 15."
+  #elif DISABLED(RESTORE_LEVELING_AFTER_G28)
+    #error "AUTO_BED_LEVELING_UBL (<=1.1.8) always has RESTORE_LEVELING_AFTER_G28 enabled. To keep this behavior, #define RESTORE_LEVELING_AFTER_G28. To keep it disabled comment out this line in SanityCheck.h."
   #else
     static_assert(WITHIN(UBL_PROBE_PT_1_X, MIN_PROBE_X, MAX_PROBE_X), "UBL_PROBE_PT_1_X can't be reached by the Z probe.");
     static_assert(WITHIN(UBL_PROBE_PT_2_X, MIN_PROBE_X, MAX_PROBE_X), "UBL_PROBE_PT_2_X can't be reached by the Z probe.");