ActionMap: Add add_action(name, slot).

This is a convenience method similar to
g_action_map_add_action_entries() in C.
Gio::SimpleAction: set_enabled(): Add default parameter value.
Glib::DateTime: Fix compare() and equal().

* glib/src/datetime.hg: Fix the conversion from const DateTime& to
gconstpointer. Bug #704639.
tools: Add docs to the default constructor of _CLASS_OPAQUE_COPYABLE.

* tools/m4/class_opaque_copyable.m4: Add documentation to the generated
default constructor, mentioning that the constructed object is invalid.
Bug #704639.
ActionMap: add_action(): Actually add the action.

Thanks to Kjell.
Glib::TypeTraits<>: Avoid warnings when compiling with -Wcast-qual.

* glib/glibmm/containerhandle_shared.h: Move const_cast<CTypeNonConst>(ptr)
from the TypeTraits< Glib::RefPtr<T> > specialization to the
TypeTraits< Glib::RefPtr<const T> > specialization, thus avoiding warnings
when compiling with -Wcast-qual. Bug #704640.
configure.ac: Avoid deprecated libsigc++ API.
Glib::Variant: Fix memory leaks.

* glib/src/variant.[hg|ccg]: Call g_variant_builder_unref(). Don't take
extra ref on objects returned from g_variant_get_child_value() or
g_variant_get_variant(). Bug #704851.
Merge branch 'master' of git://git.gnome.org/glibmm