Feature request: Add ability to continue flashing different images after first one is complete

[brs17]

Distribution (run cat /etc/os-release):
Pop!_OS 18.04

Related Application and/or Package Version (run apt policy $PACKAGE NAME):
popsicle-gtk

Issue/Bug Description:
As someone who uses popsicle quite a lot to image a number of different images, after writing an image to an external drive, I currently have to close the application and then relaunch it to image another drive with a different image.

I can imagine a button in the top left (opposite from the "Close" button) that would go back to the "choose an image" page.

[mmstick]

I'll have a PR submitted tomorrow morning. I have it working at the moment, but I would like to improve my current implementation before I submit it.