Openning other pictures with Flameshot

Hi,

I discovered this **GREAT** app a few days ago and now I only wish I could open other pictures with it just to use the great editing tools (adding arrows etc). I don't know if I'm being clear, I think it would be a great enhancement to allow flameshot to edit screeshots taken with other apps, or other pictures alltogether.

And because `gnome-capture -w` allows to take a screenshot of a window (like suggested [here](https://github.com/lupoDharkael/flameshot/issues/5)) it might create a way to take a sceenshot of only the current window by using gnome-capture. But I don't know if it's a good way.

The issue names no files, tests, or entry points. Start by tracing how Flameshot captures and loads images, then determine whether external pictures can enter the existing editing flow. Done would require an agreed scope and a verified way to open and annotate images from other applications.

Written by the indexing model from the issue text.