Fix gizmos app new panic

[pablo-lua]

Objective

After the Gizmos changes, App::init_gizmos_group turned into a important function that for sure mustn't panic. The problem is: the actual implementation causes a panic if somehow the code is runned before GizmoPlugin was added to the App

• The error occurs here for example:

fn main() {
    App::new()
        .init_gizmo_group::<MyGizmoConfig>()
        .add_plugins(DefaultPlugins)
        .run();
}

#[derive(Default, Reflect, GizmoConfigGroup)]
struct MyGizmoConfig;

Solution

• Instead of panicking when getting GizmoConfigStore, insert the store in App::init_gizmos_group if needed

---

Changelog

Changed

• Changed App::init_gizmos_group to insert the resource if it don't exist

Removed

• Removed explicit init of GizmoConfigStore

[pablo-lua]

I'm very confused about this CI error, didn't mess with bevy_ecs here 🤔

[alice-i-cecile]

Merging as trivial <3