#[class(tool)] for per-class editor run behavior

[Bromeon]

Builds on top of #365 to allow per-class granularity.

If you want a class' lifecycle callbacks to be run within the Godot editor, add the #[class(tool)] key.

#[derive(GodotClass)]
#[class(init, tool, base=Node2D)]
struct MyClass { ... }

By default, only tool classes are run in the editor now. You can still enable all classes in ExtensionLibrary, which can be useful if you are building an editor plugin rather than a game.

Also fixes a bug where on_notification() was still running by default.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-374