Split up the bevy_ui::render module

[ickshonpe]

Objective

The size and mixed-up responsibilities of the bevy_ui::render module makes the UI renderer difficult to understand.

Solution

Split up the bevy_ui::render module so that it is only responsible for setting up the render app. Move the rest of the code into new extract, prepare and queue modules.

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Changelog

bevy_ui::render

• Added extract, prepare and queue modules.
• mod.rs contains the render app setup.
• ExtractedUiNode, ExtractedUiNodes, extract_uinodes, extract_atlas_uinodes, extract_uinode_borders and extract_text_uinodes are moved to the extract module.
• prepare_uinodes, UiVertex and UiBatchare moved to theprepare` module.
• queue_uinodes is moved to the queue module.

Migration Guide

extract, prepare and queue modules have been added to bevy_ui::render.

• ExtractedUiNode, ExtractedUiNodes, extract_uinodes, extract_atlas_uinodes, extract_uinode_borders and extract_text_uinodes have been moved to the extract module.
• prepare_uinodes, UiVertex and UiBatchhave been moved to theprepare` module.
• queue_uinodes has been moved to the queue module.