Add missing window title to Help Contents dialog
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.
Example from Fedora 29:
However, on some other graphic toolkits / window managers an unspecified title is shown.
Example from GParted Live with Fluxbox:
To address the unspecified title on non-GNOME 3 graphic toolkits, add a title that works with and without GNOME 3.
Proposed dialog on Fedora 29:
Proposed dialog on Kubuntu 16.04: