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.
You can't. QSysInfo only has MacVersion on Mac. So either we use ifdefs and QSysInfo, or ifdefs and Objective-C to get the version. Which is how DolphinWX does it. I personally think this is cleaner.