Upgrade to Bevy v0.12

[thombruce]

At the time of writing, Bevy 0.12 is almost done (https://github.com/bevyengine/bevy/milestone/14) and I have to assume a release is just around the corner.

Migration guide: https://bevyengine.org/learn/migration-guides/0.11-0.12/

Let's not be too hasty though; we still have other dependencies which need to be updated to support Bevy 0.12:

• Bevy: https://bevyengine.org/news/bevy-0-12/
• bevy-inspector-egui: upgrade to bevy 0.12 jakobhellermann/bevy-inspector-egui#157
• bevy_common_assets: Update to Bevy 0.12 NiklasEi/bevy_common_assets#21
• bevy_spatial: laundmo/bevy-spatial@60cdda7
• leafwing-input-manager: Update for Bevy 0.12 Leafwing-Studios/leafwing-input-manager#408
• bevy_fluent: Update bevy requirement from 0.11.0 to 0.12.0 kgv/bevy_fluent#35 (see comment on issue)
• bevy_rapier2d: Update to Bevy 0.12 dimforge/bevy_rapier#439
• bevy_asset_loader: Update to Bevy 0.12 NiklasEi/bevy_asset_loader#158
• bevy-ui-navigation: Bump bevy to 0.12 nicopap/ui-navigation#39
• bevy_tiling_background (currently using a fork: https://github.com/rparrett/bevy_tiling_background, "tex-typo" branch)

Also note the window icon special case (bevyengine/bevy#1031) and update winit to an appropriate version if the version used by Bevy also moves. This appears not to have cahanged.

[thombruce]

Just look at those links I was able to add!! Five out of the eight crates I need to update are already on top of this. Fantastic stuff!

[thombruce]

Bevy Fluent API is expected to change before a new release. See comment: kgv/bevy_fluent#35 (comment)

kgv points to PR kgv/bevy_fluent#36 available as a "quick upgrade to Bevy 0.12", so that's a check for bevy_fluent. 👍

6 down, 4 to go.