Fix horizontal alignment with insert=fullpage

[uliska]

Fixes #227

• Only insert left-margin or inner-margin into the .ly document
• Choice depends on whether the LaTeX document is oneside or twoside
• Also, left and right margins have to be determined differently for oneside docs
• Incidentally: Improve indentation of \paper block in generated .ly code

Resulting LilyPond code

• for a onesided document:

\paper{
    #(set-paper-size "lyluatexfmt")
    print-page-number = ##f
    print-first-page-number = ##t
    first-page-number = 1
    top-margin = 68.327744\pt
    bottom-margin = 56.905502\pt
    left-margin = 71.131882\pt
    two-sided = ##f
    line-width = 369.88591003418\pt

    #(define fonts
    (make-pango-font-tree "Linux Libertine O"
                          "Linux Biolinum O"
                          "InconsolataN"
                          (/ staff-height pt 20)))

}

• for a twosided document:

  
  \paper{
  #(set-paper-size "lyluatexfmt")
  print-page-number = ##f
  print-first-page-number = ##t
  first-page-number = 2
  top-margin = 68.327744\pt
  bottom-margin = 56.905502\pt
  inner-margin = 71.131882\pt
  two-sided = ##t
  line-width = 369.88591003418\pt
  
  #(define fonts
  (make-pango-font-tree "Linux Libertine O"
                        "Linux Biolinum O"
                        "InconsolataN"
                        (/ staff-height pt 20)))

}

These are for the default `fullpagealign=crop` but I also checked with `staffline`