[FEATURE REQUEST] Display a progress bar
Well, it's nice to inform the users how much data have been written to the disk.

Currently all it does is show a spinner
I suggest that you parse stdout of the dd command at the interval of 1 second and update the progressbar. dd already says how many bytes have been written.
I would make a PR but I do not know python well enough.
I have been meaning to get around to this.
Can probably use this https://lazka.github.io/pgi-docs/Gio-2.0/classes/Subprocess.html#Gio.Subprocess.get_stderr_pipe parse the stderr and use some regex magic.
Just create a temporary file and keep parsing it like I did. https://gitlab.com/9898287/nixwriter/blob/master/source/backend.d
Nice, I like yours better than mine, I had no idea there were gtk bindings for D. I have been thinking of redoing ddgtk in Rust.