Expose constants for jCal properties?

Was originally going to expose this on the parser but one of the benefits of the new component structure is we will eventually be able to remove the dependency on the parser in the component layer (for pure jCal files) so we may not want to directly tie those.

Start by reviewing the parser and component layer to determine where jCal property constants belong and whether exposing them would preserve the intended separation. Define the public API and confirm that it avoids coupling pure jCal components to the parser before implementing or documenting the change.

Written by the indexing model from the issue text.