Allow elements on title page to be disabled by theme

[mojavelinux]

Allow individual elements on title page to be disabled by theme using display: none. For example, to disable the main title:

title_page:
  title:
    display: none

For now, the top key on title_page_title will still be honored for general positioning (until #431 is implemented).

[mojavelinux]

The motivation for this issue is to allow the title page to consist only of a background image, which may perhaps supply the whole design for the title page. But the title page still exists as a title page and is linked to from the outline.