Making this app a full featured screenshot editor

## Use case
Fork this application as a second path, a full featured screenshot editor.

## Suggested solution

As a long time Shutter user, it is getting too hard to keep up with keeping the app running with all of it's GTK2 dependencies. The developer appears to have stopped supporting it.

Suggested additions:

- Add a pre-defined "Arrow" drawing to the current set of tools.
- Add Drop Shadow effect, Border, plus any others you can think of, but those two are all you need at first.
- Add three main capture tools; Entire screen, Selection, and Window

### Possible drawbacks

More work for you.

### Possible alternatives

Currently I have resorted to using Spectacle (KDE Screenshot tool) on my Manjaro Cinnamon Desktop, because it has a great screen selection tool that remembers the last position, and then I export it to Drawing.
It works really well, just takes a few more steps on my part right now, but it is an 80% replacement for Shutter!

As for a name? Shotdrawing...Screendraw...Drawingtime...Grabon...Shotgrabber? I don't know, have a few drinks, it will come to you.

The issue names no files, tests, or entry points; begin by surveying the Python desktop application's existing drawing tools and capture flow. Done would require an agreed scope and implementation of arrow, drop shadow or border effects, and whole-screen, selection, and window capture workflows.

Written by the indexing model from the issue text.