Allow quick drawing over screenshot

## Feature
One of the main things I miss when using this app is that often I want to screenshot an entire window, but draw over a specific part of it. Currently I have to open it in GIMP/another drawing app and edit it there, which frankly is a bit annoying, not to mention overkill.

Common use case: I want to show a friend how to do something in a website/app, so I screenshot it and circle a menu option in there.

**Describe the solution you'd like**
Ideally, there could be a new shortcut accelerator that, after taking the screenshot, would open the image in a floating window that displays some basic drawing controls and allow us to draw over it. There could also be a button to open it in the default image viewer app.

**Existing work**
Snipping tool on Windows 10 offers this functionality (although the rest of the UI kinda sucks). It could be simpler than that, with just a pen option and a color picker.