Release v0.3.0

What's Changed

• Migrate to bevy 0.15.0
• Trigger observers when sending events by @patrickariel in #4
• Fix tick_time_runner_system to use Time instead of Time<Real> by @Multirious in #6
• Update cfg(CHANNEL_NIGHTLY) build script by @Multirious in #7
• Fix unused imports by @Multirious in #8
• Update tests

Full Changelog: v0.2.0...v0.3.0