Fold Slur_proto_engraver into Slur_engraver

Fold Slur_proto_engraver into Slur_engraver

A symmetrical common base class to both Slur_engraver and
Phrasing_slur_engraver seems like an unnecessary complication.  Instead,
Phrasing_slur_engraver can just be derived from Slur_engraver .


Also contains commits:

Restructure slur engravers

Replaces data members specific to derived classes of Slur_proto_engraver
with virtual functions.


internal_make_grob: Remove unused `name' argument

[dak, 2016-06-21 12:30:55]

Add lily/include/engraver.hh, massage include files

[dak, 2016-06-22 16:24:53]

While we are at it... Some more polish.

[dak, 2016-06-23 17:29:42]

Rebase on current master