Add Gtk::ListBox and Gtk::ListBoxRow.

* .gitignore: Add new generated files in gtk/gtkmm/.
* gtk/gtkmm.h: Add listbox.h and listboxrow.h.
* gtk/src/filelist.am: Add listbox.hg and listboxrow.hg.
* gtk/src/gtk_signals.defs: Regenerate with GtkListBox's and GtkListBoxRow's
signals and properties.
* tools/extra_defs_gen/generate_defs_gtk.cc: Add GTK_TYPE_LIST_BOX and
GTK_TYPE_LIST_BOX_ROW.
* tools/m4/convert_gtk.m4: Add conversions for GtkListBoxRow.
* docs/reference/images/widgets/listbox1.png:
* gtk/src/listbox.[hg|ccg]:
* gtk/src/listboxrow.[hg|ccg]: New files. Bug #708115.
ListBox: Add append() for clarity

* gtk/src/listbox.[hg|ccg]:
  Even though it is not in the GTK+ API.