Implement new handling for margin and line-width settings.

[joeneeman]

Could you add some regression tests (doesn't have to be in this commit)
that demonstrate some of the possible combinations of margin settings?
There should also be some tests that demonstrate the warnings.


http://codereview.appspot.com/104085/diff/1/4
File lily/output-def.cc (right):

http://codereview.appspot.com/104085/diff/1/4#newcode149
Line 149: if ((lookup_variable (ly_symbol2scm ("is-layout")) ==
SCM_BOOL_T) && (scm_line_width != SCM_UNDEFINED)) return;
put the return on a separate line

http://codereview.appspot.com/104085/diff/1/4#newcode183
Line 183: if (paper_width != (line_width + left_margin + right_margin))
In case there is rounding, it is better to check
if (abs (paper_width - line_width - left_margin - right_margin) > 1e-6)

http://codereview.appspot.com/104085/diff/1/4#newcode205
Line 205: scm_from_double(paper_width - left_margin_default -
right_margin_default));
indentation

http://codereview.appspot.com/104085