Allow users to extend the gallery how they want

[martinRenou]

Use case

#1071 made me think we should allow users to extend the gallery how they see fit.

Preferred solution

Maybe the preferred solution is to do that through lab settings.

A setting extra_gallery_entries which is a JSON object following the same format as our gallery entries. And we would extend the default, or even allow to overwrite it completely.

We could also provide a context menu on the layer tree allowing the workflow right click -> save into gallery to save any layer in the extra_gallery_entries setting

Alternative solutions

No response

[mfisher87]

Duplicate of #842!

[martinRenou]

My ability to forget things is amazing

[mfisher87]

I feel you 😆 It's always fun finding some code, thinking "I would do that differently", and then finding out I wrote it 🙃 It happens way too often