Log Message:
glibmm: update to 2.56.0.

2.56.0 (stable):

Glib:
* Threads::Private: Fix gobj().
  (Kjell Ahlstedt) Bug #791711 (octoploid)
* TimeoutSource: Use monotonic time consistently.
  (Kjell Ahlstedt) Bug #792524 (Dainis Jonitis)
* Add RefPtr::get().
  (Kjell Ahlstedt) Bug #495762 (Hubert Figuiere)

Gio:
* TlsClientConnection: Deprecate set/get/property_use_ssl3().
  (Kjell Ahlstedt)
* DataInputStream: Deprecate read_until().
  (Kjell Ahlstedt)
* Application: Add property_resource_base_path_string().
  This is a replacement for property_resource_base_path() which can't be
  fixed without breaking ABI.
  (Kjell Ahlstedt) Bug #787496 (Daniel Boles)
* Application: Add three set_option_context_*() methods.
  (Kjell Ahlstedt)
* DesktopAppInfo: Add get_locale_string().
  (Kjell Ahlstedt)

Documentation:
* Slightly elaborate Glib::Variant<Variant> docs.
  (Daniel Boles) Bug #778219

tests:
* glibmm_variant: Don't use C++14 features when a C++11 compiler is used.
  (Kjell Ahlstedt, Jonathan Wakely) Bug #787648 (Armin K.)
* glibmm_interface_move test: Avoid unused function warnings.
  (Murray Cumming)

gmmproc:
* _WRAP_METHOD: Accept optional list of exceptions in errthrow.
  (Kjell Ahlstedt) Bug #787979 (Daniel Boles)
* _WRAP_METHOD: Suppress the @return section in generated documentation
  if return type is void.
  (Kjell Ahlstedt) Bug #787978 (Daniel Boles)
* Add _MEMBER_SET_STR macro, setter for strings.
  (Pavlo Solntsev) Bug #793778