Notify clients when sources are added/removed/rearranged in a scene

[Palakis]

Add event types SourceOrderChanged and RepopulateSources

"SourceOrderChanged"

Scene items have been rearranged.

• "scene-name" (string) : name of the scene where items have been reordered

"SceneItemAdded"

An item has been added to the current scene.

• "scene-name" (string) : name of the scene
• "item-name" (string) : name of the item added to scene-name

"SceneItemRemoved"

An item has been removed from the current scene.

• "scene-name" (string) : name of the scene
• "item-name" (string) : name of the item removed from scene-name