Add support for user-installable applications

[VelvetToroyashi]

I've attempted to fix the failing unit tests, however it's complaining that the fields aren't surviving being serialized and then deserialized, to which I'm not entirely sure how to fix.

Issue was I didn't update the remaining samples. Fixing that has shown me that I have other things I need to add and fix.

[Nihlus]

Please rebase this onto the latest commit so you get all the required ReSharper inspection checks.

[VelvetToroyashi]

Well, messed up my commit dates, but at least it's rebased now. :v

[MazeXP]

You might want to add the [PublicAPI] attribute on the interfaces ;)