Replace RemovedComponents<T> backing with Events<Entity> (#5680)

# Objective
Removal events are unwieldy and require some knowledge of when to put systems that need to catch events for them, it is very easy to end up missing one and end up with memory leak-ish issues where you don't clean up after yourself.

## Solution
Consolidate removals with the benefits of `Events<...>` (such as double buffering and per system ticks for reading the events) and reduce the special casing of it, ideally I was hoping to move the removals to a `Resource` in the world, but that seems a bit more rough to implement/maintain because of double mutable borrowing issues.

This doesn't go the full length of change detection esque removal detection a la bevyengine/rfcs#44.
Just tries to make the current workflow a bit more user friendly so detecting removals isn't such a scheduling nightmare.

---

## Changelog
- RemovedComponents<T> is now backed by an `Events<Entity>` for the benefits of double buffering.

## Migration Guide
- Add a `mut` for `removed: RemovedComponents<T>` since we are now modifying an event reader internally.
- Iterating over removed components now requires `&mut removed_components` or `removed_components.iter()` instead of `&removed_components`.

Author: Aceeri

src/system.rs

@@ -2,7 +2,8 @@ use bevy_ecs::{
     entity::Entity,
     event::EventWriter,
     prelude::{Changed, Component, Resource},
-    system::{Commands, NonSendMut, Query, RemovedComponents},
+    removal_detection::RemovedComponents,
+    system::{Commands, NonSendMut, Query},
     world::Mut,
 };
 use bevy_utils::{
@@ -89,7 +90,7 @@ pub(crate) fn create_window<'a>(
 pub struct WindowTitleCache(HashMap<Entity, String>);
 
 pub(crate) fn despawn_window(
-    closed: RemovedComponents<Window>,
+    mut closed: RemovedComponents<Window>,
     window_entities: Query<&Window>,
     mut close_events: EventWriter<WindowClosed>,
     mut winit_windows: NonSendMut<WinitWindows>,