Add app links similar to external pages

Hello,

additional apps installed into Nextcloud should be shown as items in the menu of the mobile app just like the external pages it shows now. As it already shows web pages, this would be handy to show additional Nextcloud apps that are already shown in the browser on mobile/desktop.

This would remove the requirement to switch to a browser from the Nextcloud app instead of listing and displaying them directly inside the official Nextcloud app.

Additionally, a new handler could be created that allows dedicated companion apps for Android that interface directly with certain Nextcloud apps (like for example a Bookmarks app or Notes app) so an Android menu shows up which app to use/start (either the web app inside the Nextcloud app or the dedicated one).