Allow user to choose the default ui camera

[pablo-lua]

What problem does this solve or what need does it fill?

There is an example of a problem that this can lead in #11237, where we cannot spawn a new Camera in the same window as primary window without possible problems appearing (Debug camera interpolating with Ui camera)

What solution would you like?

• The creation of a marker Component DefaultUiCamera (or a similar name: This name is already in use and can lead to conflicts), so when selecting the camera with a root entity that doesn't have TargetCamera, the DefaultCamera param will give priority to this component.

What alternative(s) have you considered?

• The user, when creating a camera for debug purposes (Or another purposes that for now I don't know if exists), will have to add an TargetCamera for each root that exists (even though only one camera will load the Ui, the other Camera is created for another purposes) (This option Don't need the API Refactor)
• Return with the UiCameraOptions, an component that marks if the Camera should show ui or not (I think we don't need this anymore)

Aditional Context

• For now, say that you have two cameras spawned for PrimaryWindow -> The actual API will not return a warn or something, it will almost randomly pick one of the cameras. The camera depends on the camera order, the line is as follow:

    cameras: Query<'w, 's, (Entity, &'static Camera)>,

///...
self.cameras
    .iter()
    .filter(|(_, c)| match c.target {
        RenderTarget::Window(WindowRef::Primary) => true,
        RenderTarget::Window(WindowRef::Entity(w)) => self.primary_window.get(w).is_ok(),
        _ => false,
    })
    .max_by_key(|(e, c)| (c.order, *e))
    .map(|(e, _)| e)

but using the order can conflict if the user is building an debug camera (That generally will be above others)

[nicoburns]

Could this also be fixed by not having a default ui camera at all? I feel like it might lead to a clearer, more explicit API if you just always need to declare a camera for each UI root.

[pablo-lua]

I was aware of this approach, but if the TargetCamera is a required component (you Always have to declare it in the root), so we might as well put it in the NodeBundle or in other Bundles just so the User never forget to put this Component.
I really think that its ok to have a DefaultCamera though