install desktop file, icons and manpage with install-linux target

And add a separate uninstall-linux target.
I am not too sure if this way of getting the relative path is good practice, as it depends on perl, which, yes, is installed on most machines (and is probably even a requirement to build ngale at its current state, I recall there being perl scripts), but especially since the conventions say to avoid using perl (though then we shouldn't have used xdg- commands in the first place...): https://www.gnu.org/prep/standards/html_node/Utilities-in-Makefiles.html#Utilities-in-Makefiles.