In the case where we deselect the previously selected font (e.g.
because we've just changed the filter settings to remove it from the
list), we were leaving the preview pane in its previous state, which
is fine in GTK2 when it just carries on displaying the last thing
drawn to the backing pixmap but goes wrong in GTK3 where we still have
to actually respond to draw events.
But it makes more conceptual sense anyway to actually empty the
preview pane when no font is selected, so now we do that. So now
unifontsel_draw_preview_text() is called from unifontsel_deselect(),
and also the preview-drawing code will still draw the background
rectangle regardless of whether font != NULL.
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