Changes to glChess [Gnome Games]

["Robert Ancell" <robert ancell gmail com>]

Hi all,

There have been a few changes to glChess to fix some important
features that are tripping up users.

1. A pop-up dialog has been added that tells the user what needs to be
installed to enable 3D. (#389371)
2. AI difficulty has been added - the default AI is GNUchess which is
very hard to beat (#388259)

The result is:

1. There is a large string describing why 3D does not work by default
which needs to be translated.
2. There are a number of new short strings describing game difficulty.
3. Also an unused dialog has been removed to avoid it being
unnecessarily translated; this has removed.

Anyone who tries it out please tell me if you think the AI difficulty
is appropriate, you can change the options in the ai.xml file (copy
one to ~/.gnome2/glchess).

Thanks,
--Robert