Bdbag-gui

Experimental GUI for [bdbag](https://github.com/ini-bdds/bdbag).

Release Notes:

* Updated to use official BDBag 1.1.3 release.

Experimental GUI for [bdbag](https://github.com/ini-bdds/bdbag).

Release Notes:

* Updated to use official BDBag 1.1.1 release.
* Added version information for GUI, `bdbag` and `bagit` in About dialog box.

BDBag GUI beta release.

MacOS (BDBag.dmg) and Windows (BDBag.zip) binaries provided.