Make `Visibility` optional in most built-in systems

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

Many built-in systems (_e.g._ [`extract_text2d_sprite()`](https://docs.rs/bevy/latest/bevy/text/fn.extract_text2d_sprite.html) query a core component they want to update (_e.g._ `Text`) with an associated unconditional `Visibility` component. If removed, the system doesn't run on the `Entity`. However, most components (and especially the ones directly related to rendering) should be assumed visible by default. The ability to dynamically control the visibility of a render item should be an additional opt-in feature, not a default one.

## What solution would you like?

Replace `Visibility` with `Option` in the query of most built-in systems, and treat `None` as `Visibility::is_visible == true`.

```rust
if !maybe_visibility.map_or(true, |vis| vis.is_visible) {
continue; // skip this entity
}
```

## What alternative(s) have you considered?

Do nothing. This forces adding many unused `Visibility` components for all entities that are never going to be hidden (they might be deleted, but not dynamically shown/hidden).

## Additional context

The more "mandatory" components in a built-in system, the more difficult it is to reuse the built-in Bevy components. I'm trying to use `Text` to render some text, but with some properties (notably, the position, but also the visibility) controlled by my own framework. I could go rewrite from scratch a text rendering pipeline, but it feels like reusing the existing one is probably the best course of action. However currently the [`extract_text2d_sprite()`](https://docs.rs/bevy/latest/bevy/text/fn.extract_text2d_sprite.html) system forces adding 3 extra components in addition of `Text`, which I argue is too much and at least `Visibility` should be optional (if not others; alignment could also be made optional for the cases where there's no need to align).