Introduce new handling for paper margin settings.

Introduce new handling for paper margin settings.

* Make default margins accessible in ly/paper-defaults-init.ly

* Introduce a new method: Output_def::normalize (). It checks,
  whether left-margin, right-margin and/or line-width are set.
  Afterwards it computes missing values or takes defaults, if
  necessary. There is no need to specify line-width manually now.

* Make right-margin work

* If only line-width is set, the current behaviour persists.
  (Systems are centered)

* Output a warning if the margin values don't fit with each other or
  systems run off the page due to improper settings

* Modify lilypond-book to set a default line-width in case there
  is no other to base on left-padding (This is ugly, though)