* src/glade-app.c: reworked glade_app_command_paste () placeholders can
  not be selected anymore.

* src/glade-command.c: in glade_command_cut_copy_paste_common () we try
  to use the placeholder which has the pointer over it first.

* src/glade-fixed.[ch]: add new property "use-placeholders".

* src/glade-gtk.c: set "use-placeholders" for GtkBox and GtkTable.

* src/glade-popup.c: reworked glade_popup_create_placeholder_menu () and
  glade_popup_placeholder_paste_cb (), placeholders do not get selected
  anymore.

* src/glade-utils.[ch] glade_util_selected_placeholder () removed,
  added glade_util_get_placeholder_from_pointer ().