created dir.c & .h, improved header file handling

file_previews.c

@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include "defines.h"
+#include "backend.h"
 
 static FILE *ueberzug = NULL;
 extern unsigned int terminal_height;