* scm/lily-library.scm (interval-translate): new function

* scm/lily-library.scm (interval-translate): new function
(interval-center): new function.

* scm/page-layout.scm (paper-system-annotate): new function. Add
arrows for dimensions.

* scm/stencil.scm (dimension-arrows): new function.

* Documentation/user/global.itely (Vertical spacing): mention annotatespacing

* input/regression/page-spacing.ly: add annotatespacing

* lily/paper-system-scheme.cc (LY_DEFINE): remove ly:paper-system-{extent,stencil}
(LY_DEFINE): new function ly:paper-system-set-property!

* Documentation/user/global.itely (Paper size): explain how to add
sizes.