DolphinQt: and so it begins

This adds the beginning of the DolphinQt user interface. It doesn't do anything useful yet and only builds via CMake.
I agree that private stuff should be `m_`, but can we make an exception for `ui`? Naming it `ui` is Qt convention (literally, I don't think you can find a project that doesn't use this name) and I'm not sure if automatic slot connection will work without it.