Use reverse DNS name for desktop file and icon

[ColinKinloch]

Description

This MR renames the svg icon and desktop files to reverse DNS com.libretro.RetroArch.
This brings the desktop file in line with the FDO desktop file spec: https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#file-naming

I have not tested this in any great detail. I'm not sure whether renaming these files will break anything.

This is the case for the Flatpak.

The fedora package uses this for the desktop file but not the icon: https://packages.fedoraproject.org/pkgs/retroarch/retroarch/fedora-40.html#files

This isn't the case for the arch linux package: https://archlinux.org/packages/extra/x86_64/retroarch/

Related Issues

#14521 (comment)

[RobLoach]

Should be good. I don't see any other references to retroarch.desktop. I messed up the name when making the Flatpak build 🙃