awesome: add optional gtk3 support

Add optional gtk3 support to Awesome so that the `beautiful.gtk` module can be
used.

The `beautiful.gtk` uses `lgi` to obtain Gtk via gobject-introspect:

    return require('lgi').Gtk

Since the current build does not include the typelib files needed, the above
call fails.

It turns out that both `gtk3` and `atk` (Accessibility toolkit) are needed, so
this commit adds them as optional build inputs.

Setting `gtk3Support` to `true` e.g. in an overlay will make `beautiful.gtk`
work at the cost of an increased closure size (currently 99.6M vs 223.4M).

Fixes https://github.com/NixOS/nixpkgs/issues/60538