Add missing window title to Help Contents dialog (!45)

When GParted is configured with `--disable-doc` the help documentation
is neither built, nor installed.

With this configuration the Help -> Contents menu displays a message
dialog indicating the "Documentation is not available".

On GNOME 3 no title is shown; however, on some other graphic toolkits
/ window managers an unspecified title is shown.

For example on GParted Live with Fluxbox the following is displayed:

                         Unnamed
                         -------

               Documentation is not available

      This build of gparted is configured without documentation.
      Documentation is available at the project web site.
      https://gparted.org

                           OK

To address the unspecified title on non-GNOME 3 graphic toolkits, add
a title that works with and without GNOME 3.

Closes !45 - Add missing window title to Help Contents dialog
4 jobs for add-missing-help-contents-window-title in 16 minutes and 36 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #324525
centos_build

00:05:34

passed #324526
ubuntu_build

00:03:46

 
  Test
passed #324527
centos_test

00:11:02

passed #324528
ubuntu_test

00:08:58