Ensure that plugin entry points are always exported on macOS and Linux (otherwise, plugins won't load in macOS release builds)

* Added template files for FileSource and DataThread plugin types.
* Minor stylystic fixes and refactoring for some FileSource plugins.

* Refactoring and stylystic fixes of RecordEngine class (and one
  dependency).
* Refactoring and stylystic fixes of DataThread class and all
  appropriate dependent classes.

Created Xcode projects for all currently-built plugins.  Added Xcode workspace with "All" scheme for building GUI and all plugins.  Addressed some compiler warnings in plugin code.