Move exo_gtk_dialog_get_action_area() to libxfceui and remove exo_gtk_dialog_add_secondary_button()
I just looked up if there is any other `G_GNUC_BEGIN_IGNORE_DEPRECATIONS` hidden, and there is one: `exo_gtk_dialog_get_action_area()` is a wrapper to turn off the deprecation warning. ```c GtkWidget * exo_gtk_dialog_get_action_area (GtkDialog *dialog) { G_GNUC_BEGIN_IGNORE_DEPRECATIONS return gtk_dialog_get_action_area (dialog); G_GNUC_END_IGNORE_DEPRECATIONS } ``` `thunar_column_editor_init()` and `exo_gtk_dialog_add_secondary_button()` use this one, but the latter isn't used anywhere within Xfce project. So the only part that use this function is Thunar. However, `glade_xfce_titled_dialog_post_create()` in libxfce4ui has the same implementation of this [(kind of)](https://gitlab.xfce.org/xfce/libxfce4ui/-/blob/40743c4158f4f4d2357dd72326a915a969f20f94/glade/libxfce4ui-glade.c#L110) so it would be better to move this into libxfce4ui.
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