Only run event systems if they have tangible work to do

[james7132]

Objective

Scheduling low cost systems has significant overhead due to task pool contention and the extra machinery to schedule and run them. Event update systems are the prime example of a low cost system, requiring a guaranteed O(1) operation, and there are a lot of them.

Solution

Add a run condition to every event system so they only run when there is an event in either of it's two internal Vecs.

[github-actions[bot]]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy? It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.