Consider support for different styles of footnote in the same file

[windymilla]

It's possible that Roman footnotes might be per-chapter, restarting the numbering for each chapter, and that Arabic footnotes are actually endnotes at the end of the book.

Currently (based on #503) this isn't possible in GG2. However it does appear as though some books require it. A workaround of changing the Arabic (for example) to {1} and {Footnote 1: text} so the Arabics are ignored while you deal with the Roman, and then vice versa, is possible, but a little cumbersome.

GG1 also supports changing the style of individual footnotes, but I'm not sure why/if this is needed.

[G4OEU]

Most/all the points here are addressed by suggestions in the comment to #526.