Long-lasting external streams

[bdlukaa]

The problem

Previously, it was possible to add external streams to the "External Streams" layout only. On the latest release, it is now possible to add external streams to any layout by clicking the "Add external stream" icon button. This works great, but if the user wants to reutilize these external streams with a defined configuration, they have to add them every time and set it up.

Proposal

Add an option to define external streams in the sidebar, alongside the servers and devices. This way streams will be able to be added to any layout and share the same options as the other streams.

Additional context

https://forums.bluecherrydvr.com/t/feature-requests-for-v3-client/962