SD Card Alpha Sorting

First iteration of alphabetical sorting for SD cards, both
slow+efficient and fast+rammy. Option for folders to sort first, last,
or not at all.

Marlin/SdFatConfig.h

@@ -111,10 +111,12 @@ uint8_t const SOFT_SPI_SCK_PIN = 13;
 /**
  * Defines for long (vfat) filenames
  */
+/** Number of UTF-16 characters per entry */
+#define FILENAME_LENGTH 13
 /** Number of VFAT entries used. Every entry has 13 UTF-16 characters */
 #define MAX_VFAT_ENTRIES (2)
 /** Total size of the buffer used to store the long filenames */
-#define LONG_FILENAME_LENGTH (13*MAX_VFAT_ENTRIES+1)
+#define LONG_FILENAME_LENGTH (FILENAME_LENGTH*MAX_VFAT_ENTRIES+1)
 #endif  // SdFatConfig_h