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This should have been done before releasing 1.8.0

Should have done this when introduced the use of
exo_gtk_dialog_get_action_area.

Requires libxfce4ui >=4.13.2

The thumbnailer code is a bit hairy, but it appears to work.
Let's wait for the crashes and bug reports.

Optional plugins are not ported yet. We use a shitload of
deprecated Gtk APIs now. Not all features work.
It doesn't look great yet. There are bugs.

But, well, it's a first step.