shell: Fix title bar when

Before migrating to HdyHeaderBar, gtk_window_set_title was enough to set the title in the window and GtkHeaderBar because GTK internally synced both. That bevahiour does not occur with HdyHeaderBar, thus HdyHeaderBar was updating the title/subtitle only when the document and the filename were set, leaving the title in blank otherwise That is, when both title_header and subtitle were set, and did nothing otherwise.

We now check than in any other situation, we fallback to the title, which would take a proper value if there is no document, or only we have its file name.

Fixes #1583 (closed)