[Feature] Portable mode and Bakup/Restore

Hello and thanks for such a great app.

Portable mode is a nice feature that makes the app save all settings in its own folder instead of the user folder (`C:\Users\UserName\.abdm`), so the settings and downloads list won't be affected by reinstalling Windows, We can also put the app on a USB drive. We can enable this mode by creating an empty Portable.dat next to ABDownloadManager.exe, which the app checks for on startup, if found, enables Portable mode, if not, continues as usual.

By Backup/Restore I mean a feature to create a backup of your download list (like IDM). It should be a plain text file so we can restore it in other download managers and also restore the downloads list of other apps in AB Download Manager.