Improve value retriever/comparer registration

[szaliszali]

Fixes #1218

I think there is too much similarity in comparer and retriever handling in the Assist service, so I propose they could be merged into one with a small API change:

Before:

Service.Instance.RegisterValueComparer(___)

After:

Service.Instance.ValueComparers.Register(___)

Additions:

Service.Instance.ValueComparers.Register<MyClass>()
Service.Instance.ValueComparers.Unregister<MyClass>()
Service.Instance.ValueComparers.Replace(oldObj, newObj)
Service.Instance.ValueComparers.Replace<OldClass, NewClass>()

Pros:

• simplifies future maintenance

Cons:

• breaking change
• RegisterDefault only applied to comparers in the past (and it's weird to add more "default"s to a single list but I did not change this behavior)

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added an entry to the changelog

[kevinkuszyk]

Posting a note here to highlight #1267 also has a fix for #1218 (I had already written the code before I saw this PR).

Only one of these two PRs should be merged.

[SabotageAndi]

I like the idea with the ServiceComponentList class, so I think I prefer this PR over #1267.
@szaliszali Could you add an entry in the changelog for this.

[szaliszali]

rebased the branch on current master and force pushed to solve merge conflict without the mess