Use more const Grobs in the Paper_column interface (issue 346100043 by address@hidden)

[dak]

https://codereview.appspot.com/346100043/diff/1/lily/paper-column.cc
File lily/paper-column.cc (right):

https://codereview.appspot.com/346100043/diff/1/lily/paper-column.cc#newcode105
lily/paper-column.cc:105: SCM m = me->get_property ("when");
How much sense does this make?  get_property on callbacks will execute
various sorts of callbacks the first time a property is accessed and
store a cached result.  So the grob is only "const" because accessing
its properties happens via a linked list where only the data structure
organization rather than the data itself happens to affect the formal
Grob constness.  Which is sort of an implementation detail.

https://codereview.appspot.com/346100043/