Add a system to pass messages to the UI / User

[benthillerkus]

I know there are some ways already implemented, but it would be nice if there was some kind of general method to do it: maybe a property called status, or function that you could call to inform the user on what's going on. Info might include:

• Progress (Downloading 3/5 leaves_roughness_4K.png), (Searching provider etc)
• Errors (No provider found), (No results found), (Wrong type...)
• Info (Added textures, downloaded textures to), (Asking to support the creators of these textures, license info)
• Easier usage for debugging

It would be best if the scrappers had pretty much free hand in deciding what to show, to allow for lots of different types of scrappers.