enhance: gui_local_slot_editing - collection of suggested improvements

[LukasGold]

• Make purging of folder optional / or note when downloading
• Set data folder when loading page as would do when downloading slots! (otherwise, an upload of the slots from a previous run, which was not uploaded, is not possible)
• make uploading / json parsing error proof -> catch exception to avoid crashing of the app -> display error messages
• allow the gui to start even with empty accounts.pwd
• create (by default) a folder within the data folder that replicates the source-domain