Frame_Glom: Use GtkStack instead of our PlaceHolder widget for data/find modes.

* glom/frame_glom.[h|cc]: Replace the Glom::PlaceHolder with
  GtkStack, using set/get_visible_child() instead of adding, removing
  and getting the GtkBox child. This is simpler and safer.
* glom/mode_data/box_data_details.[h|cc]:
  Constructor: Use show_all_children() instead of show(),
  to avoid showing this when the parent widget does not want it.
  Also override show_all_vfunc() to make sure that we never show
  the layout toolbar accidentally.

This simplifies the code, so we don't have to worry about side-effects
of repeatedly adding and removing widgets from their container.