Add Glib::Binding

* .gitignore: Ignore binding.[h|cc].
* glib/glibmm/objectbase.h: Add a comment.
* glib/src/filelist.am: Add glib_signals.defs and binding.hg.
* glib/src/glib.defs:  Add glib_signals.defs.
* glib/glibmm.h: Add binding.h.
* tools/extra_defs_gen/generate_defs_glib.cc: Add get_defs(G_TYPE_BINDING).
* tools/m4/convert_glib.m4: Add conversion for BindingFlags.
* tools/generate_wrap_init.pl.in: Define GLIBMM_INCLUDED_FROM_WRAP_INIT_CC.
* glib/src/binding.[hg|ccg]: New files with enum BindingFlags and class Binding.
* glib/src/glib_signals.defs: New file with info about GBinding's properties.
Bug #738663.