* scm/layout-page-layout.scm (page-breaking-wrapper): new

* scm/layout-page-layout.scm (page-breaking-wrapper): new
function. Call the page breaking function selected in the
`page-breaking' \paper variable, then the post processing function
chosen using the `page-post-process' \paper variable.
(line-height): new function. Return the height of a system.
(line-minimum-position-on-page): new function. Return the position
of a system on page (using the previous line position), only
considering between system padding.
(stretchable-line?): new function. Says whether a line can be
stretched (ie. is not a title nor a single staff system).
(page-maximum-space-left): new function. Computes space left on a
page, when all systems are separated by their padding.

* lily/page-breaking.cc (breaking::make_pages): Move page post
processing function call to page breaking wrapper (common to all
page breakers).

* lily/paper-book.cc (book::pages): call the page breaking
wrapper, instead of the page breaker directly

* ly/paper-defaults.ly: Add \paper variables for page breaking
wrapper and page post processing function. Make
`write-page-layout' value depend on the 'dump-tweaks option. Add a
`system-maximum-stretch-procedure' variable for holding a function
computing the maximum stretch a system allows.

* scm/layout-page-dump.scm (write-page-breaks): computes the
stretch to apply to systems on a page to minimize left
space. Dump this stretch length.

* ly/music-functions-init.ly (spacingTweaks): implement it. Read
the system-stretch property of the tweak data to stretch the
system.
(includePageLayoutFile): Void function which includes the
generated page-layout file if it exists and if the page layout
dumping is not asked.
(scoreTweak): if the score tweak named by the argument exists,
return it.