App actions do not work on demos using the external previewer

Thanks to @UrtsiSantsi for noticing this.

Currently all the demos running in an out-of-process previewer (Vala, Rust, Python) do not have a GtkApplication associated with it. Because of this actions that require the app (such as `app.open_uri`) in menus etc. do not work and are grayed out.

This was noticed in #705 and #714.

