Insert option: "fullpage" with arbitrary page size

[uliska]

This issue supersedes #268 in order to distill the essence of that request without the noise. There may be thoughts to consider in that issue, though.
@lemzwerg

In order to overcome a (seemingly unsolvable) limitation in lilypond-book-preamble.ly we should add an option to the insert= option that engraves a "fullpage" score with an arbitrary smaller paper size. Fullpage compilation is currently the only way to preserve LilyPond's vertical page layout, i.e. the spacing between systems, and in certain cases the results of insert=systems are unacceptably poor.

TODO (partially after more consideration/discussion):

• Add a new insertion mode. Name suggestions: score, page, smallpage (I'd withdraw the other suggestions from Wanted: 'insert=fullpage' for tables #268 (comment))
• Do not adjust the page margins to the .tex document's margins.
• However, provide options to set custom page margins.
• Create a paper size. As default value I suggest the current \textwidth and a height calculated from the DIN ratio (width*sqrt(2)), but that could be discussed.
  It should be discussed whether it is useful to investigate \pagetotal for a default value of the height (I seriously doubt this will work in floating environments for us).
• Provide an option page-staffsize (again, corresponding to the final name), that - similarly to inline-staffsize can globally control the staffsize in these smaller scores.
• Other options (fonts, indent, etc.) should be passed along as usual.

I suggest to add one option (e.g. page-options, or anything corresponding to the final name of the insertion mode) rather than individual options for all configurable parameters. This option will then be parsed internally.

While the original request in #268 will only make sense with scores that are limited to one page I would not in any way limit the functionality in that direction because there are other use cases where multi-page scores can be useful as well, e.g. when creating "thumbnail"-like situations or a table with mulitple score pages placed on one paper page. Probably these would be handled with raw-pdf but I think we can simply ignore this for the implementation. When a user tries to put a two-page score in a figure environment they'll notice the issue themselves ;-)