cmake-gui: Fix desktop file icon configuration
Drop the icon file extension.  This fixes the desktop file validation
message:
 "CMakeSetup32.png" for key "Icon" in group "Desktop Entry" is an icon
 name with an extension, but there should be no extension as described in
 the Icon Theme Specification if the value is not an absolute path
Applied-by: Rolf Eike Beer <[email protected]>