This replaces the const ContentType.DEFAULT_CONTENT_TYPE string and
related code with static defaults for both displayed and attached MIME
entities.

* src/engine/mime/mime-content-type.vala (ContentType): Replace
  DEFAULT_CONTENT_TYPE const with static DISPLAY_DEFAULT and
  ATTACHMENT_DEFAULT static instances. Remove is_default() since that no
  longer makes sense. Update call sites and unit tests.

* src/engine/mime/mime-content-parameters.vala (ContentParameters): Make
  the class immutable so it is safe to be used as a component of the
  static ContentType DISPLAY_DEFAULT and ATTACHMENT_DEFAULT members.