The custom install script still depended on GTK 3.
Instead of updating it, let’s just use Meson’s purpose-built functions.

The FIXME in the install script was already resolved by
7ed2ff45

Also let’s run `update-desktop-database` since we install a desktop file.

This requires Meson 0.59 for `post_install`’s `update_desktop_database` kwarg,
and Meson 0.61 for `install_symlink`.