fixed desktop file in install script

[ronnystandtke]

I fixed the following issues in the desktop file:

• the name should not contain the version number (not interesting in the desktop menu, deprecates too fast)
• at least one category should be added (I added "Development")
• the entry "Icon" doesn't support quoting and completely fails to load the icon when used, therefore I had to removed it
• the PNG icon has a much better resolution so let's use it instead of the small and blurry ICO version