Untrack entity when double-clicking away

[adridavid]

Fixes #5959

This modifies the viewer default input action for the LEFT_DOUBLE_CLICK event type.
If an entity is being tracked by the viewer, double-clicking away from it (except on another entity) will untrack the entity.

[cesium-concierge]

Thank you so much for the pull request @adridavid! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
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[pjcozzi]

Thanks again @adridavid, we received your CLA.

[hpinkos]

Thanks @adridavid! This is a change I've wanted to make for a while.

[hpinkos]

Added a note to CHANGES here: d6c7110

[emackey]

This is a good change, thanks @adridavid and @hpinkos