Backward-compatible unserialization to v0.7.3

[sdatkinson]

Thanks for making a Pull Request!
Please fill out this template so that you can be sure that your PR does everything it needs to be accepted.

Description

Older versions of the plugin back to 0.7.3 (maybe 0.7.0) unserialize into a reasonable state for the current version.
Resolves #526.

PR Checklist

• Did you format your code using format.bash?
• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• Does your PR add, remove, or rename any plugin parameters?
  • If yes, then have you ensured that older versions of the plug-in load correctly? (Usually, this means writing a new legacy unserialization function like _UnserializeStateLegacy_0_7_9.)