Allow adding an action block to scenes via external integration

Allow adding one or more new action blocks to scenes provided by an external integration (declare a kind of function library).

Hi @jean_bruder!

Since Gladys 5.1, an external integration can add its own blocks to the scene editor.

It declares its triggers and actions. They appear in the block selection, alongside Gladys’s, and are used in the same way. An integration can thus propose « run such a command » without manually adding it to the core.

I’m categorizing this topic under « Delivered Requests ». If there’s still a case that doesn’t fit, open a new topic: it will be easier to follow.