Issue 4478: Disentangle some Tie and Semi_tie code.

Both are Grobs, but ties are Spanners and semi-ties are Items.  The code for both cases was jumbled together in many Tie methods.  I hope that I have clarified a few things.  No functional changes are intended.

[Dan Eble, 2015-07-12 00:27:14]

https://codereview.appspot.com/251840044/diff/1/lily/tie.cc
File lily/tie.cc (right):

https://codereview.appspot.com/251840044/diff/1/lily/tie.cc#newcode128
lily/tie.cc:128: one_head = Semi_tie::head (item);
Whoops.  This should have been Semi_tie::head (item, d).  Obviously the
regression tests didn't catch this mistake.  I will try to find input that shows
that this matters (and probably touches on some of the questions I raised
below).

[Dan Eble, 2015-07-12 12:22:16]

On 2015/07/12 00:27:14, Dan Eble wrote:
> lily/tie.cc:128: one_head = Semi_tie::head (item);
> Whoops.  This should have been Semi_tie::head (item, d).

I am almost sure this code is reachable only under conditions that are detected
as programming errors elsewhere.  Therefore I am not going to worry about
restoring it now, as it should disappear as my clean-up work continues.

[Dan Eble, 2015-07-14 11:38:05]

The next spurt of work is in https://codereview.appspot.com/254090043/