align-interface.cc: Clarify code for empty staves

While tentatively evaluating potential line-breaks for planning of page-filling, the old code removed tentatively-empty axis-groups from an array in one function and restored them in another.
This patch keeps the array intact.

[Keith, 2013-09-25 07:01:17]

https://codereview.appspot.com/13768045/diff/1/lily/align-interface.cc
File lily/align-interface.cc (left):

https://codereview.appspot.com/13768045/diff/1/lily/align-interface.cc#oldcod...
lily/align-interface.cc:216: else if (!last_nonempty_element)
{
 for (int k = j; k--; ) translates[k] = dy * stacking_dir;

To match the ver2.16 code, bringing any empty lines to the same position as the
first full line.  (Better, though, if the code that estimated system height did
not depend on the positions of empty lines.)

https://codereview.appspot.com/13768045/diff/1/lily/align-interface.cc#oldcod...
lily/align-interface.cc:263: if (Page_layout_problem::is_spaceable (elems[j]))
  && ! skyline.is_empty() )
Doh.  We did not want to remember the previous Staff if it was empty.

Probably best to have the if-elsif-else construction above fill the whole for
loop, because this code after that construction is better done separately for
the if, elseif, and else situations.