Draft: Optionally show gtk header bar

This MR is not to be merged (at least not now), but rather to discuss how optional GtkHeaderBar should be implemented in general in Xfce.

For the record: Connecting to "notify::gtk-dialogs-use-header" to change the GtkHeaderBar during runtime wont work. That would cause:

(xfce4-taskmanager:37947): Gtk-WARNING **: 23:43:38.309: gtk_window_set_titlebar() called on a realized window

... and the window is moved to the top-left corner of the screen.