Feature/private symbols

[lanther]

What does this PR do?

Updating Disposable/Signal/SignalDispatcher/Observable so they use symbols for their private state - since these all add to classes that users create, the more we can avoid conflicting with existing properties the better.

Checklist

• [ ] I've read the contribution guidelines and the Code of Conduct
• [ ] An issue has been created for this PR
• [ ] Title begins with one of the following: Fix:, Update:, Breaking:, or New:
• [ ] Title ends with a reference to the related issue(s): (fixes #issue) or (refs #issue)
• [ ] Linting and unit tests all pass (npm test)
• [ ] PR includes new unit tests as necessary (or will be submitted as a separate PR)
• [ ] PR includes documentation as necessary (or will be forthcoming in a separate PR)