doc: Re-add the section about print preview, updated for GTK3

doc/geany.txt

@@ -3216,6 +3216,23 @@ same as the ones which can be used with the ANSI C strftime function.
 
 All of these settings can also be changed in the print dialog just before
 actual printing is done.
+On Unix-like systems the provided print dialog offers a print preview. The
+preview file is opened with a PDF viewer and by default GTK uses ``evince``
+for print preview. If you have not installed evince or just want to use
+another PDF viewer, you can change the program to use in the file
+``settings.ini`` (usually found in ``~/.config/gtk-3.0``, see the
+`GTK documentation`_). For example, use::
+
+    [Settings]
+    gtk-print-preview-command = epdfview %f
+
+Of course, you can also use xpdf, kpdf or whatever as the print preview
+command.  That command should ideally delete the temporary file referenced by
+``%f``.  See the `GTK documentation for the setting`_ for more details.
+
+.. _GTK documentation: https://developer.gnome.org/gtk3/stable/GtkSettings.html#GtkSettings.description
+.. _GTK documentation for the setting: https://developer.gnome.org/gtk3/stable/GtkSettings.html#GtkSettings--gtk-print-preview-command
+
 
 Geany also provides an alternative basic printing support using a custom
 print command. However, the printed document contains no syntax highlighting.