Switch from GTK to a Rust native GUI

[qskousen]

It would be nice to switch to a Rust native GUI at some point, which would simplify the build and package process quite a bit. Potential crates are egui, Dioxus, and Iced.

The only reason we can't switch now is because they all rely on on the same base window crate, and that crate doesn't yet have the ability to be a drag and drop source. We wouldn't be able to drag images out of StableKeepr into, say, a web browser or discord - a pretty common task.

I'm watching relevant issues and pull requests on the base window crate, and will look into the feasibility of switching when that feature is completed.

[qskousen]

Trying to use GTK was an exercise in frustration, and pretty much stalled the project. According to this, winit is getting closer to having drag and drop support, which should enable it in the GUI libraries mentioned above. As such, I'm pivoting from GTK to Iced.