The goal and scope of the GTK backend in druid-shell

Nobody has claimed this yet.

What exactly is the goal of the GTK backend in druid-shell?

Is it a temporary solution or a more permanent one?
- If temporary, what is the real goal? Some sort of barebones X11 and Wayland support?
- If it's a permanent target, then what is the scope?
- Ubuntu
- All Linux flavors (distros/X11/Wayland)
- All GTK platforms (including Windows/macOS)

The problem with GTK is that it's not consistent. It has platform specific behavior. For example for mouse events the [ModifierType docs](https://valadoc.org/gdk-3.0/Gdk.ModifierType.html) state *the GDK X backend interprets button press events for button 4-7 as scroll events, so `gdk_button4_mask` and `gdk_button5_mask` will never be set*. There are also similar issues with keyboard events.

This means that it's not possible to just target one GTK. The GTK druid-shell layer has to basically have an internal gtk-shell abstraction which deals with delivering a consistent GTK API across the different platforms that GTK might run on.

Start by reading the GTK backend in druid-shell and its handling of platform-specific mouse and keyboard behavior. Clarify whether the backend is temporary or permanent, which GTK platforms it should support, and whether a consistent internal abstraction is required; done means the scope and goal are documented and agreed.

Written by the indexing model from the issue text.