adjust multi-file search to make it support searching in multiple directories

Also, add a button that populates the directories from collections and
refactor it to make the "configure search" dialog easier to work with,
ie so it can be run without starting up drracket

pkgs/drracket-pkgs/drracket/drracket/private/main.rkt

@@ -580,49 +580,6 @@
   
   (preferences:set-default 'drracket:show-killed-dialog #t boolean?)
   
-  (drr:set-default 'drracket:multi-file-search:recur? #t boolean?)
-  (drr:set-default 'drracket:multi-file-search:filter? #t boolean?)
-  (drr:set-default 'drracket:multi-file-search:filter-regexp "\\.(rkt.?|scrbl|ss|scm)$" string?)
-  (drr:set-default 'drracket:multi-file-search:search-string "" string?)
-  (drr:set-default 'drracket:multi-file-search:search-type
-                           1
-                           (λ (x) 
-                             (and (number? x)
-                                  (exact? x)
-                                  (integer? x)
-                                  (<= 0 x)
-                                  (< x (length drracket:multi-file-search:search-types)))))
-  
-  ;; drracket:mult-file-search:search-check-boxes : (listof (listof boolean))
-  (drr:set-default 'drracket:multi-file-search:search-check-boxes 
-                           (map (λ (x) (map cdr (drracket:multi-file-search:search-type-params x)))
-                                drracket:multi-file-search:search-types)
-                           (λ (x) 
-                             (and (list? x)
-                                  (andmap (λ (x)
-                                            (and (list? x)
-                                                 (andmap boolean? x)))
-                                          x))))
-  
-  (drr:set-default 'drracket:multi-file-search:percentages
-                           '(1/3 2/3)
-                           (λ (x) (and (list? x)
-                                       (= 2 (length x))
-                                       (= 1 (apply + x)))))
-  
-  (drr:set-default 'drracket:multi-file-search:frame-size '(300 . 400) 
-                           (λ (x) (and (pair? x)
-                                       (number? (car x))
-                                       (number? (cdr x)))))
-  (drr:set-default 'drracket:multi-file-search:directories 
-                   '()
-                   (lambda (x) (and (list? x) (andmap string? x))))
-  (drr:set-default 'drracket:multi-file-search:directory 
-                           ;; The default is #f because
-                           ;; filesystem-root-list is expensive under Windows
-                           #f 
-                           (lambda (x) (or (not x) (string? x))))
-
   
   (drr:set-default 'drracket:large-letters-font #f (λ (x)
                                                        (or (and (pair? x)