[mojosetup] set the working path for the desktop entry

[Ryan C. Gordon]

> It's on the TODO list.

Fwiw, I just pushed support for .tar.xz files, which aren't 7-Zip, but 
use the same LZMA compression that 7-Zip does.

7-Zip's SDK isn't useful for our purposes; the C version wants to 
allocate memory for the entire file you're uncompressing, so if you want 
to install a multi-gigabyte file, you're in big trouble. The C++ version 
apparently doesn't have this issue, but we can't use it, as we can't 
accept a dependency on the C++ runtime.

liblzma from XZ Utils (which I used for the .xz support) is actually a 
much nicer API, and much better to use with apps like MojoSetup. I'll 
probably write up some code to parse a 7-Zip file and use liblzma for 
the hard work of decompressing them, when I have time to do so.

--ryan.