Add run condition for whether egui is using pointer input

[jakobhellermann]

It came up in jakobhellermann/bevy-inspector-egui#147 that it is often useful to run some systems (like looking around) only if egui is not using the pointer input.

mod common_conditions {
  fn egui_mouse_unused(egui_contexts: Query<&EguiContext>) -> bool {
      egui_contexts
          .iter()
          .all(|ctx| !ctx.get().wants_pointer_input())
  }
}

app.add_systems(
    Update,
    (camera_pan, camera_zoom).run_if(bevy_egui::common_conditions::mouse_pointer_valid()),
)

For prior art, bevy also has some common_conditions for various subcrates:

• https://docs.rs/bevy/latest/bevy/time/common_conditions/index.html
• https://docs.rs/bevy/latest/bevy/input/common_conditions/index.html
• https://docs.rs/bevy/latest/bevy/ecs/schedule/common_conditions/index.html