Potential display perf improvements

[sebastienros]

While looking at display events I noticed some potential improvements

• PlacementInfo and its results (tab/group/alternates) could be reused since they don't change when created from a string source
• ShapeDescriptor uses collections that might be empty. Check if we can lazy-load them. Note that they should be already reused across tenants so maybe it's not much gain at least for the cost of instantiation, not memory usage which would still be gained.
• Alternate collections like the ones for content parts in ContentPartDisplayDriver<TPart> should remain constant across usages. Maybe these could be cached per part. Or at least used as an immutable collection such that other parts of the code can be based on it. Another option is to have a list of collections so one can be reused while others can be added dynamically.

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