Function on_enter_signal 

pub fn on_enter_signal(
    cmd: Commands<'_, '_>,
    text: Single<'_, '_, &mut Text, With<StateText>>,
    target_segment: Res<'_, TargetSegment>,
    assets_manager: Res<'_, AssetsManager>,
)

Handles going into EditorState::Signal, spawning the position indicator and changing the state text.