Show the progress of the backup status, e.g. via a progress bar

When backing up large files or directories, it may be nice to have a rough overview of required time and disk space. This could be realized by calculating the size of the data and estimate it's transfer time and compression ratio.

After doing several backups, BIT could even remember the last compression rates and transfer times and do a better estimation.

Backup status could be shown in a progress bar, showing the transfered data in percent and the elapsed as well as the estimated time. This progress bar could be shown in the current status bar, so no great UI changes were necessary.