Make separate structure for apt and cloudlab screenshots

In CloudLab mode, uses the Apt version if the CloudLab one is missing,
but emits a warning.

defs.rkt

@@ -72,7 +72,13 @@
                      properties))))
 
 (define (screenshot path)
-  (let* ([fullpath (string-append "screenshots/" path)]
+  (let* ([aptpath (string-append "screenshots/apt/" path)]
+         [clabpath (string-append "screenshots/clab/" path)]
+         [fullpath (if (apt?)
+                     aptpath
+                     (if (file-exists? clabpath)
+                       clabpath
+                       (cdr (cons (displayln (string-append "WARNING: CloudLab missing screenshot " path)) aptpath))))]
          [b (make-object bitmap% fullpath)]
          [width (send b get-width)]
          [scale-factor (/ screenshot-width width)])