New UI for blivet-gui

I started working on blivet-gui more than a year ago. The UI was originally mostly inspired by GParted -- its UI and visualization of partitions is simple and easy to understand. Displaying devices and their relations this way works quite nice for "simple" storage configurations (e.g. only partitions or basic LVM setup) but it's really hard to understand (and to work with) for more "complex" storage configurations that involves RAIDs end encryption.