Rename apply_system_buffers to apply_deferred

[alice-i-cecile]

Objective

• apply_system_buffers is an unhelpful name: it introduces a new internal-only concept
• this is particularly rough for beginners as reasoning about how commands work is a critical stumbling block

Solution

• rename apply_system_buffers to the more descriptive apply_deferred
• rename related fields, arguments and methods in the internals fo bevy_ecs for consistency
• update the docs

Changelog

apply_system_buffers has been renamed to apply_deferred, to more clearly communicate its intent and relation to Deferred system parameters like Commands.

Migration Guide

• apply_system_buffers has been renamed to apply_deferred
• the apply_system_buffers method on the System trait has been renamed to apply_deferred
• the is_apply_system_buffers function has been replaced by is_apply_deferred
• Executor::set_apply_final_buffers is now Executor::set_apply_final_deferred
• Schedule::apply_system_buffers is now Schedule::apply_deferred

[github-actions]

You added a new example but didn't update the readme. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[iiYese]

I think this file name needs to be changed.

[iiYese]

Change example module name to reflect renaming.

[JoJoJet]

Good change. I've always found it difficult to write succinct documentation surrounding system buffers.

[github-actions]

You added a new example but didn't update the readme. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[github-actions]

You added a new example but didn't update the readme. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[JoJoJet]

LGTM, just a quick question though.