Make ComponentTicks available from ChangeTrackers

[chrisjuchem]

Objective

To implement indexes, I need to be able to check if a component has changed since an arbitrary time, not just since the last time the system ran.

Solution

Allow for getting a read only reference to ComponentTicks from ChangeTrackers so that ComponentTicks::is_added and ComponentTicks::is_changed can be used, since they take the last_change_tick as a parameter.

Alternatives

Implement is_added_since and is_changed_since on ChangeTrackers itself, deferring to ComponentTicks under the hood.

Changelog

• Added ticks method to ChangeTrackers to get the ComponentTicks tracking when a component was added/changed.

[alice-i-cecile]

It feels like the DetectChanges trait is the right place for this functionality, rather than special-casing this for ChangeTrackers.

[chrisjuchem]

Makes sense, I'll move it there.

[JoJoJet]

FYI, ChangeTrackers will probably get removed (see #7306). So this code from bevy_mod_index:

type ChangedComponetsQuery<'w, 's, T> = Query<
    'w,
    's,
    (
        Entity,
        // TODO: Figure out if the static lifetime is right here
        &'static <T as IndexInfo>::Component,
        ChangeTrackers<<T as IndexInfo>::Component>,
    ),
>;

Would become this:

type ChangedComponetsQuery<'w, 's, T> = Query<
    'w,
    's,
    (
        Entity,
        Ref<'static, <T as IndexInfo>::Component>,
    ),
>;

Also: the static lifetime is indeed correct here. The lifetimes bound to WorldQuery types don't really matter -- WorldQuery::Item<'w> is where the lifetimes start to matter.

[chrisjuchem]

Thank you for the pointers @JoJoJet.

It turns out that DetectChanges actually already provides last_changed which was enough for me to implement what I needed, so this PR is no longer needed.