scene: Allow dropping assets from the tree into the scene editor

[elisee]

We should register a plugin that associates an asset type with an action ("create an actor and put a SpriterRenderer component with this sprite asset on it", "create a prefab with this scene asset", etc.)

[elisee]

The framework is in place and importing prefabs and sprites works!

What's left to do:

• Place the newly created actor at the location where the mouse is pointing
• Select the actor after its been created
• Allow importing all the existing assets that make sense