Gtk::Stack and StackSwitcher: Add wrapper

* gtk/gtkmm.h: add stack.h and stackswitcher.h
* gtk/src/enums.hg: wrap StackTransitionType
* gtk/src/filelist.am: add stack.hg stackswitcher.hg
* gtk/src/gtk_signals.defs: Regenerate
* gtk/src/stack.ccg:
* gtk/src/stack.hg:
* gtk/src/stackswitcher.ccg:
* gtk/src/stackswitcher.hg:
* tools/extra_defs_gen/generate_defs_gtk.cc: Add stack and stackswitcher
* tools/m4/convert_gtk.m4: Add conversions to/from Stack*

Bug #704964