implement case-insensitive sort, make default and configurable

This will convert any file name to lowercase before doing the comparison.
Note that this will only work for ASCII character range, a full Unicode
aware solution will be much more complicated. And in the end, file names
are byte arrays, not character strings ;-)

fixes #1183.

defaults.lua

@@ -155,6 +155,12 @@ DDICT_FONT_SIZE = 20
 -- e.g. 2 changes the sensitivity by 1/2, 3 by 1/3 etc.
 FRONTLIGHT_SENSITIVITY_DECREASE = 2
 
+-- Normally, Koreader will present file lists sorted in case insensitive manner
+-- when presenting an alphatically sorted list. So the Order is "A, b, C, d".
+-- You can switch to a case sensitive sort ("A", "C", "b", "d") by disabling
+-- insensitive sort
+DALPHA_SORT_CASE_INSENSITIVE = true
+
 -- Set a path to a folder that is filled by Calibre (must contain the file metadata.calibre)
 -- e.g.
 -- "/mnt/sd/.hidden" for Kobo with files in ".hidden" on the SD card

frontend/ui/widget/filechooser.lua

@@ -73,8 +73,14 @@ function FileChooser:genItemTableFromPath(path)
     local sorting = nil
     local reverse = self.reverse_collate
     if self.collate == "strcoll" then
-        sorting = function(a, b)
-            return self.strcoll(a.name, b.name) == not reverse
+        if DALPHA_SORT_CASE_INSENSITIVE then
+            sorting = function(a, b)
+                return self.strcoll(string.lower(a.name), string.lower(b.name)) == not reverse
+            end
+        else
+            sorting = function(a, b)
+                return self.strcoll(a.name, b.name) == not reverse
+            end
         end
     elseif self.collate == "access" then
         sorting = function(a, b)