Refactor settings page with IContainerBaseWithNamedChildren

[sdatkinson]

Description

Add helper class that extends iplug::igraphics::IContainerBase to enable working with child controls by name instead of by index that's assigned when they're added.

PR Checklist

• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• [N] Does your PR add, remove, or rename any plugin parameters?
  • [N/A] 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.)