Lightweight status screen for ST7920

- This status screen uses the ST7920 character generator to greatly
  reduce SPI traffic and MCU load when updating the status screen.

- Has been tested with the RepRapDiscount Full Graphics Smart Controller
  but should work with any LCD that uses an ST7920 or fully compatible
  controller.

Marlin/src/config/default/Configuration_adv.h

@@ -656,6 +656,22 @@
   // Swap the CW/CCW indicators in the graphics overlay
   //#define OVERLAY_GFX_REVERSE
 
+  #if ENABLED(U8GLIB_ST7920)
+    /**
+     * ST7920-based LCDs can emulate a 16 x 4 character display using
+     * the ST7920 character-generator for very fast screen updates.
+     * Enable LIGHTWEIGHT_UI to use this special display mode.
+     *
+     * Since LIGHTWEIGHT_UI has limited space, the position and status
+     * message occupy the same line. Set STATUS_EXPIRE_SECONDS to the
+     * length of time to display the status message before clearing.
+     */
+    //#define LIGHTWEIGHT_UI
+    #if ENABLED(LIGHTWEIGHT_UI)
+      #define STATUS_EXPIRE_SECONDS 20
+    #endif
+  #endif
+
 #endif // DOGLCD
 
 // @section safety