Footer: fix items possibly missing (#6212)

Iterating a k/v table as an array would have us stopped
on the first disabled MODE (frontlight, battery... on
some devices).
Rework that initial setup, and make it correctly handle
added or removed MODEs when using the ordered items
saved in the user settings.

frontend/apps/reader/modules/readerfooter.lua

@@ -326,10 +326,7 @@ function ReaderFooter:init()
         text_font_bold = false,
     }
 
-    local mode_tbl = {}
-    local mode_name
-    self.mode_nb = 0
-    self.mode_index = {}
+    -- Remove items not supported by the current device
     if not Device:isAndroid() then
         MODE.wifi_status = nil
     end
@@ -339,23 +336,43 @@ function ReaderFooter:init()
     if Device:isDesktop() then
         MODE.battery = nil
     end
-    for k, v in pairs(MODE) do
-        mode_tbl[v] = k
-    end
-    for i = 0, #mode_tbl do
-        mode_name = mode_tbl[i]
-        if mode_name then
-            self.mode_index[self.mode_nb] = mode_name
-            self.mode_nb = self.mode_nb + 1
-        end
-    end
+
+    -- self.mode_index will be an array of MODE names, with an additional element
+    -- with key 0 for "off", which feels a bit strange but seems to work...
+    -- (The same is true for self.settings.order which is saved in settings.)
+    self.mode_index = {}
+    self.mode_nb = 0
+
+    local handled_modes = {}
     if self.settings.order then
-        while #self.settings.order < #self.mode_index do
-            self.settings.order[#self.settings.order + 1] = self.mode_index[#self.settings.order + 1]
+        -- Start filling self.mode_index from what's been ordered by the user and saved
+        for i=0, #self.settings.order do
+            local name = self.settings.order[i]
+            -- (if name has been removed from our supported MODEs: ignore it)
+            if MODE[name] then -- this mode still exists
+                self.mode_index[self.mode_nb] = name
+                self.mode_nb = self.mode_nb + 1
+                handled_modes[name] = true
+            end
+        end
+        -- go on completing it with remaining new modes in MODE
+    end
+    -- If no previous self.settings.order, fill mode_index with what's in MODE
+    -- in the original indices order
+    local orig_indexes = {}
+    local orig_indexes_to_name = {}
+    for name, orig_index in pairs(MODE) do
+        if not handled_modes[name] then
+            table.insert(orig_indexes, orig_index)
+            orig_indexes_to_name[orig_index] = name
         end
-        self.mode_index = self.settings.order
-        self.mode_nb = #self.mode_index
     end
+    table.sort(orig_indexes)
+    for i = 1, #orig_indexes do
+        self.mode_index[self.mode_nb] = orig_indexes_to_name[orig_indexes[i]]
+        self.mode_nb = self.mode_nb + 1
+    end
+    -- require("logger").dbg(self.mode_nb, self.mode_index)
 
     -- default margin (like self.horizontal_margin)
     if not self.settings.progress_margin_width  then