[feat] Synchronize local folder with dropbox (#5591)

Option to synchronize files in local koreader folder with folder on Dropbox. All files that aren't on the local disk will be downloaded. Files with different sizes will also be downloaded and overwritten.
The download process can be paused or stopped at any time (similar to downloading images in Wikipedia).

Synchronize and settings are available after long press in dropbox account in Cloud storage.
Limitations: Only one folder (without subfolders) can be synchronize with one folder on local storage.

frontend/apps/cloudstorage/dropboxapi.lua

@@ -90,7 +90,9 @@ function DropBoxApi:downloadFile(path, token, local_path)
     return code_return
 end
 
-function DropBoxApi:listFolder(path, token)
+-- folder_mode - set to true when we want to see only folder.
+-- We see also extra folder "Long-press to select current directory" at the beginning.
+function DropBoxApi:listFolder(path, token, folder_mode)
     local dropbox_list = {}
     local dropbox_file = {}
     local tag, text
@@ -101,6 +103,7 @@ function DropBoxApi:listFolder(path, token)
         tag = files[".tag"]
         if tag == "folder" then
             text = text .. "/"
+            if folder_mode then tag = "folder_long_press" end
             table.insert(dropbox_list, {
                 text = text,
                 url = files.path_display,
@@ -108,7 +111,7 @@ function DropBoxApi:listFolder(path, token)
             })
         --show only file with supported formats
         elseif tag == "file" and (DocumentRegistry:hasProvider(text)
-            or G_reader_settings:isTrue("show_unsupported")) then
+            or G_reader_settings:isTrue("show_unsupported")) and not folder_mode then
             table.insert(dropbox_file, {
                 text = text,
                 url = files.path_display,
@@ -123,6 +126,14 @@ function DropBoxApi:listFolder(path, token)
     table.sort(dropbox_file, function(v1,v2)
         return v1.text < v2.text
     end)
+    -- Add special folder.
+    if folder_mode then
+        table.insert(dropbox_list, 1, {
+            text = _("Long-press to select current directory"),
+            url = path,
+            type = "folder_long_press",
+        })
+    end
     for _, files in ipairs(dropbox_file) do
         table.insert(dropbox_list, {
             text = files.text,
@@ -133,4 +144,23 @@ function DropBoxApi:listFolder(path, token)
     return dropbox_list
 end
 
+function DropBoxApi:showFiles(path, token)
+    local dropbox_files = {}
+    local tag, text
+    local ls_dropbox = self:fetchListFolders(path, token)
+    if ls_dropbox == nil or ls_dropbox.entries == nil then return false end
+    for _, files in ipairs(ls_dropbox.entries) do
+        text = files.name
+        tag = files[".tag"]
+        if tag == "file" and (DocumentRegistry:hasProvider(text) or G_reader_settings:isTrue("show_unsupported")) then
+            table.insert(dropbox_files, {
+                text = text,
+                url = files.path_display,
+                size = files.size,
+            })
+        end
+    end
+    return dropbox_files
+end
+
 return DropBoxApi