gui: Add ellipsis to menu items that require further input.

The GNOME Human Interface Guidelines say:

    Label the menu item with a trailing ellipsis ("...") only if
    the command requires further input from the user before it
    can be performed. Do not add an ellipsis to items that only
    present a confirmation dialog (such as Delete), or that do
    not require further input (such as Properties, Preferences or
    About).

Most PSPPIRE menu items fit this description, so they should have
trailing ellipses, which this commit adds.

Some GTK+ programs use the Unicode HORIZONTAL ELLIPSIS (U+2026)
character instead of three periods in menu items.  In a quick check
on my own machine, however, three periods were over twice as common.
In an informal survey on pspp-dev, the best argument presented in
favor of one or the other was that three periods may be more familiar
for translators and thus cause less confusion in translation.