main: Use the header bar to set the window title

https://bugzilla.gnome.org/show_bug.cgi?id=726324
main: Fix empty window title in some cases

Store the TotemMainToolbar title and subtitle in the GtkHeaderBar
instead of our own structure. This means that GtkWindow's call
to gtk_header_bar_get_title() would get the correct value, and not
an empty string when coming back from playing a film.

https://bugzilla.gnome.org/show_bug.cgi?id=726324
main: Fix fullscreen header bar's subtitle

It's not currently used, but could be in the future.