Upgrade Bevy to 0.15

[snendev]

Issue

This upgrades Bevy to 0.15.

Changes

• Included the required bevy_window feature in dependencies
• Renamed App::observe to App::add_observer
• Renamed Condition(?)::and_then to ::and
• Added repeat field to keyboard inputs
• World::flush_commnads is now private, replaced with World::flush
• Updated gamepad example to accommodate refactors around (1) gamepads (now entities instead of tracked by resources) and (2) Text. This comprises the bulk of the meaningful changes.

Hope this helps!

[alice-i-cecile]

Fantastic. Thanks for all your help here; I've sent you an invite for maintainer privileges on Github for this crate. I'm not actively using it, so it's nice to share the load and get real user feedback.

[alice-i-cecile]

Changes look good, but this needs release notes. Can you bump the crate version number in this PR too so we can release right after this is merged?

[snendev]

🤦🏻 my bad, I've forgotten release notes every time. can do!