[RFC] Viewer 4.0 API

Let's discuss and plan the new Viewer handling.
cc @juliushaertl @susnux @R0Wi @ariselseng @artonge @max-nextcloud

Challenges

With the new Files app and the new Files/Folder/Node API, we finally have a proper and official way of dealing with files
It's time to re-draft the Viewer and clean the various issues we encountered over the last years

Requirements

Viewer

• The Viewer is a component that can mount itself within a Modal to render a single or an array of Files
• The Viewer can also be requested to be mounted within a specified div to render a single file only. In that case there would be no interaction possible (zooming, slideshow, editing, actions.... are disabled)
• The Viewer can also be used to render two files next to each others, to compare them

Handlers

Apps can register their own views, called "Handlers"
A handler provide the following

• A unique id
• A display name
• A render function to process the current file
• A open function to change the node while keeping the handler mounted
• A toggle function to show/hide the handler (for preloading, like videos or images)
  ❓ Is this still the right approach?
• A condition to which the handler would work for the provided file (true/false)