Simplify the FontIDAllocator a lot (#2827).

Simplify the FontIDAllocator a lot (#2827).

This is at the expense of forward compatibility, and the need to
re-examine subtitle content (losing custom fonts as we do so).

But it does mean that the code is simpler, and there's not this weird
growth of IDs where a DCP gets imported with some font, and then the ID
becomes 0_font, and if you do it again it's 0_0_font, and so on.