add shortcut to add local calibre OPDS catalog directly by host and port

And there is also a "set download directory" button in the button table when you want to download a file from opds catalogs. In the "choose download directory" dialog long press on a directory will popup a confirm dialog which can be used to confirm setting the download directory variable `download_dir` in the global reader setting (the settings.reader.lua file created in the root directory in koreader).

This PR is to fix #882 and substitute #883.