Added the same icons which are for Windows - in src/windows_installer/icon.ico.
They have higher quality than previous. I also added icon 128x128 as default.
It looks much better in Gnome 3.x and probably other desktop environmetns.
git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@13606 178a84e3-b1eb-0310-8ba1-8eac791a3b58
