Closes#3689
cmake: Add `desktop-install` and `icon-install` targets. `runtime`
target will trigger them.
Specification:
https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#recognized-keys
Icons are stored system-wide in /usr/share/applications or user wide at
/usr/share/icons/hicolor/scalable/apps and can be overriden in ~/.local/share/icons
nvim.desktop file can be installed system wide or in
~/.local/share/applications/
To test without an installer:
$ xdg-desktop-menu install --novendor runtime/nvim.desktop
$ xdg-icon-resource install --novendor --mode user --size 64 contrib/nvim-icon.png
Once it is installed, you can test with gtk-launch if installed or
dmenu/rofi (drun mode)
