[UX] FileManager: add ConfirmBox for purge .sdr

Notified of destructive action without confirmation in https://www.mobileread.com/forums/showthread.php?p=3566978#post3566978

frontend/apps/filemanager/filemanager.lua

@@ -141,6 +141,11 @@ function FileManager:init()
                     text = _("Purge .sdr"),
                     enabled = DocSettings:hasSidecarFile(util.realpath(file)),
                     callback = function()
+                        local ConfirmBox = require("ui/widget/confirmbox")
+                        UIManager:show(ConfirmBox:new{
+                            text = util.template(_("Purge .sdr to reset settings for this document?\n\n%1"), self.file_dialog.title),
+                            ok_text = _("Purge"),
+                            ok_callback = function()
                                 local file_abs_path = util.realpath(file)
                                 if file_abs_path then
                                     local autoremove_deleted_items_from_history = G_reader_settings:readSetting("autoremove_deleted_items_from_history") or false
@@ -157,6 +162,8 @@ function FileManager:init()
                                 end
                                 UIManager:close(self.file_dialog)
                             end,
+                        })
+                    end,
                 },
             },
             {