Use titles in frame nodes

Frames nodes are great for organizing all the spaghetti in the shader editor, but they have one small annoying problem: they can be dragged from anywhere. We can't just click and drag to select. It also means any accidental mis-click shuffles the entire frame around.

It would be better if frames had a title area, and only that part was draggable. It would make them fit better with the design of the rest of the nodes and look more organized too. I also made it left-aligned in the mockup to look more like node titles.