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 <eike@sf-mail.de>
Merge topic 'desktop-icon' into next

0c4c29ed cmake-gui: Fix desktop file icon configuration