Issue 4142: Make skipTypesetting skip time signatures

Setting Score.skipTypesetting used to defer creating a time signature
rather than skip it entirely.  With this change, the temporal
placement of time signatures is not affected by Score.skipTypesetting.

The effect is evident in a few regression tests that set
Score.skipTypesetting at the beginning.

[Dan Eble, 2014-09-30 12:32:26]

The problem was discovered while debugging
https://codereview.appspot.com/143450043/ .

[dak, 2014-09-30 12:43:17]

https://codereview.appspot.com/148360043/diff/1/lily/time-signature-engraver.cc
File lily/time-signature-engraver.cc (right):

https://codereview.appspot.com/148360043/diff/1/lily/time-signature-engraver....
lily/time-signature-engraver.cc:63: reallocated in the same spot */
The comment is rubbish since the value is protected in the derived_mark
procedure and thus cannot get collected.  Of course, it was rubbish before, but
if we are changing the code anyway, we need not keep it.