Added observers for SubmitText and ModifyText

[Sigma-dev]

Observers have made writing UI much better, so I thought I'd add the observers I've needed when using text inputs.

If you don't like the implementation tell me and I will change it.

[PPakalns]

I would suggest removal of text field.

If text is very long, raising EntityEvents on each change with text when text is modified is costly.

Instead users can retrieve text from the text edit entity manually when entity event is raised and text retrieval is needed.

[PPakalns]

This avoids a lot of other changes that can be now reverted in this PR.

[MuongKimhong]

How is this PR? I also want to use observers.

[coolcatcoder]

While we wait for the fate of this pull request, we can use this snippet to get us observers in the mean time:

#[derive(EntityEvent)]
pub struct TextInputModified {
    #[event_target]
    pub text_input: Entity,
}

impl TextInputModified {
    pub fn plugin(app: &mut App) {
        app.add_systems(
            Update,
            |text_inputs: Query<Entity, Changed<TextInputContents>>, mut commands: Commands| {
                for text_input in text_inputs {
                    commands.trigger(TextInputModified { text_input });
                }
            },
        );
    }
}

(Don't forget to add TextInputModified::plugin to your App, and make sure to add TextInputContents to your text input entities.)