Add Epic Games provider

[JMcKiern]

This PR adds an Epic Games provider to automatically populate games installed with Epic Games. Implements part of #671

I basically just copied the code for the Steam provider so feel free to make suggestions - I'm not too familiar with modern C++.

I still need to add some metadata parser but I'm struggling with using the info in the manifest file to retrieve the metadata. I will investigate when I get some time.

Perhaps it's worth cleaning this up and merging without the metadata provider? Let me know.

[mmatyas]

Nice work! Yeah, I'm okay with a game-only initial implementation, we can add the details later. Let me know when it's ready for review.

Some notes:

• You can use / for paths
• You can use JSON parsing for the item files
• This will probably pick up Unreal Engine and its installed plugins too, but it's OK for now

[mmatyas]

Do you plan to continue working on this? I could try to finish it.

[JMcKiern]

Sorry, been busy with other things. I probably won't get around to this any time soon so feel free to finish it if you'd like to.