Level Bed in Prepare submenu

- Add “Level Bed” menu item for auto bed leveling
- Hide the option if homing has not been done yet
- Arrange the Prepare submenu more logically (?)
- Add documentation comments, some white-space
- Apply some coding standards here and there
- Move old encoder multiplier debug option to `ultralcd.cpp`

Marlin/dogm_lcd_implementation.h

@@ -172,6 +172,7 @@ char lcd_printPGM(const char* str) {
 
 static bool show_splashscreen = true;
 
+/* Warning: This function is called from interrupt context */
 static void lcd_implementation_init() {
 
   #ifdef LCD_PIN_BL // Enable LCD backlight