cmake: install desktop files and icons on FreeBSD

[vishwin]

Description

Copy the desktop file and icon routine from os-linux.cmake so that FreeBSD gets these installed too.

Motivation and Context

FreeBSD can generally run the same desktop environments that use these files as Linux. Without these files, icons and application launchers do not populate in these desktops. These files were previously installed prior to the CMake refactor here.

How Has This Been Tested?

Built as usual on FreeBSD-CURRENT.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• [ ] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.