UI texture slicing should support responsive insets values

[ickshonpe]

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

UI texture slicing should support responsive inset values.

What solution would you like?

Change the insets value in NodeImageMode::Sliced to a UiRect:

    ImageNode {
        image: asset_server.load("textures/fantasy_ui_borders/panel-border-010.png"),
        image_mode: NodeImageMode::Sliced {
            insets: UiRect::all(Val::Percent(10.)),
            center_scale_mode: SliceScaleMode::Stretch,
            sides_scale_mode: SliceScaleMode::Stretch,
            max_corner_scale: 1.0,
        }
        ..default()
    }