[mojosetup] set the working path for the desktop entry

[Ryan C. Gordon]

> Have you another idea to do that?

I just added support for this:

     http://hg.icculus.org/icculus/mojosetup/rev/35969605f674

Update your source tree, and then in your config.lua, add this to your 
Setup.DesktopMenuItem:

    workingdir = "%0"

(%0 will translate to the installation folder, so if you installed to 
/home/user/mygame and want the Path to end up being 
"/home/user/mygame/data", you'd specify "%0/data" here.)

If you don't care about the workingdir setting, you can omit it. Every 
thing I've shipped has figured out its installation directory at 
runtime, so I haven't needed this before.

This is 100% untested. It definitely compiles, though!  :)

Please let me know if this solves your problem.

--ryan.