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.
FYI: I bumped the CMake minimum version because in 2.8.8, they refactored the Qt5 detection and we'd have to add a bunch of nasty stuff to support anything lower than this. Considering we've been past 2.8.8 for a long time, this shouldn't be a problem.