* scm/stencil.scm (stack-lines): return empty-stencil if argument

* scm/stencil.scm (stack-lines): return empty-stencil if argument
is '()
(stack-stencils): idem.

* Documentation/user/changing-defaults.itely (Creating titles):
document new title layout options.

* lily/parser.yy (lilypond_header_body): copy previous
$globalheader if present.

* lily/ly-module.cc (LY_DEFINE): rename ly:import-module to
ly:module-copy

* scm/page-layout.scm (marked-up-headfoot): remove old functions.

* scm/titling.scm: remove old titling functions

* ly/titling-init.ly (oddFooterMarkup): new file. Generate titles
via markup.

* scm/page-layout.scm (marked-up-headfoot): create header/footer
field from user-supplied markup

* scm/define-markup-commands.scm (on-the-fly): new markup
command. Enter SCM markup procedure directly in Scheme.
(fromproperty): new markup command. Read markup from props argument.

* scm/titling.scm (marked-up-title): create title via
user-specified markup.

* scm/define-markup-commands.scm (column): remove empty stencils
from column.

* lily/ly-module.cc (LY_DEFINE): use ly_module_lookup(). This does
not have side-effect of creating variable stub.