Add GtkContainer's child properties handling

	* gtk/gtkmm/childpropertyproxy_base.[h|cc]: New files.
	* gtk/gtkmm/childpropertyproxy.[h|cc]: New files.
	* gtk/gtkmm/filelist.am: Add childpropertyproxy_base.[h|cc] and
	childpropertyproxy.[h|cc].
	* gtk/src/container.hg: Include gtkmm/childpropertyproxy.h.
	* tools/extra_defs_gen/generate_defs_gtk.cc: Add
	get_child_properties(), and use it to gather information about
	child properties in GtkContainer subclasses.
	* tools/m4/child_property.m4: New file that expands
	_CHILD_PROPERTY_PROXY macro.
	* tools/m4/convert_gtkmm.m4: Include child_property.m4.
	* tools/m4/filelist.am: Add child_property.m4.