Team themes shipped with the engine.

Put .otlthx files here (made with the save_theme tool, or the add-in's Themes window; export_theme
copies one out of My Themes). Every plugin that receives this engine copies them into
Documents\Lucen Timeline\Themes ("My Themes") the next time a theme tool runs:
  - new themes are installed;
  - a newer version of the same theme (same Id inside the file) replaces the installed one,
    the old file goes to %LOCALAPPDATA%\OfficeTimelineMCP\backup;
  - a user's own theme that happens to have the same name is never touched.
The file name is the theme's name.
