Update to Bevy 0.15

[Azorlogh]

This is based on Iced 0.14 which is currently unreleased.

It also required two upstream changes to Iced so I pointed this to a fork for now:

• Make iced_wgpu::Renderer Send by using Arc in text/triangle caches iced-rs/iced#2692
• Let the user of iced_wgpu::Renderer manually finish/recall the staging belt iced-rs/iced#2693

[Azorlogh]

WASM doesn't work for now. Seems bevy relies on the fragile-send-sync-non-atomic-wasm from wgpu, which lets it implement/require Send/Sync bounds. But iced doesn't use it and instead disables those bounds on wasm.
I tried adding the bounds to iced as a hack and that mostly works. But for some reason text is invisible T_T.
Text in the tour example in iced seems to work fine on webgpu.
I wanted to try the integration example as well, but unfortunately that one just hangs.