Graphical user interface

Added an (optional bu default) graphical user interface to the app. The GUI is built using the cross-platform library iced. The GUI support all modes of operations as the command line interface (direct, configs, backups, incremental backups, merging backups, restoring, and extracting files from a backup).

Another new feature is the merging of two backup files. This enables further space savings with incremental backups.