Upgrade to Bevy 0.11

[afonsolage]

PR to track changes from Bevy 0.10 to 0.11.

I'll upgrade to 0.12, but I would like to first upgrade to 0.11.

TODO:

• [x] Upgrade code base
• [x] Fix theme example
• [x] Fix hot_reload example
• [x] Fix simple_ui example
• [x] Fix alpha example
• [x] Fix stress_test example
• [x] Fix clippy warnings
• [x] Fix tests
• [x] Update docs