New icns loading code

[Fred Kiefer]

I just committed a change to gui that adds some basic icns loading even 
when libicns is not present on a system. Even with that code in place 
libicns will be the better solution in many case.
Why not just use libicns?
- It is published with the GPL 2 licence, which may not be suitable for 
some projects using GNUstep.
- It is currently not shipped with my Linux distribution.
My implementation has been done without looking at the libicns code, 
just by following there documentation and Gregs usage of their 
functions. I also added some ideas found in Nikolaus Schallers myStep 
implementation of icns loading.

The code I added has many limitations, which are probably not even worth 
fixing, as we should be using libicns for the more complex cases. There 
are also a few bugs (something that looks like an of by one case for 128 
bit images), these surely need fixing.

I also changes Gregs loading code a bit, as it was leaking memory and 
could not load any icns file that didn't have a 48x48 representation. 
Somebody needs to check that this still works with libicns.

Fred