* lily/book.cc (process): bugfix: flip ?: cases.

* lily/book.cc (process): bugfix: flip ?: cases.

* Documentation/user/changing-defaults.itely (Difficult tweaks):
add outputProperty.

* ly/music-functions-init.ly: add outputProperty music function.

* scm/paper.scm (set-paper-dimension-variables): add pagetopspace

* scm/page-layout.scm (ly:optimal-page-breaks): read next-space
and next-padding.
(optimal-page-breaks): rename from ly:optimal-page-breaks.

* lily/paper-system-scheme.cc (LY_DEFINE): new function.

* lily/paper-system.cc (internal_get_property): new function.

* Documentation/user/global.itely (Vertical spacing): refer to page-spacing.ly

* scm/page-layout.scm (ly:optimal-page-breaks): add support for
pagetopspace

* input/regression/page-spacing.ly: new file.

* input/regression/page-top-space.ly: new file.

* lily/paper-system.cc (read_left_bound): new function. Read
line-break-system-details from left bound to determine extents.

* Documentation/user/programming-interface.itely (Using LilyPond
syntax inside Scheme): change applyxxx -> applyXxx.

* ly/music-functions-init.ly: add outputProperty music function.

* ly/music-functions-init.ly: applyxxx -> applyXxx