Glib::ObjectBase: overload get_property()
Glib::ObjectBase: overload get_property()

* glib/glibmm/objectbase.h: add convenient overload of get_property(),
  which gives a property value as a return value, not as an output
  parameter.

https://bugzilla.gnome.org/show_bug.cgi?id=771782

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm