[NODES] Add UI to switch a node's "time" path in the KeyframeView/CurveView

[itsmattkc]

Originally opened as https://github.com/sobotka/olive/issues/103

Summary A node used in multiple clips may go through more than one time transform before getting to the viewer. Since keyframes are all in "viewer time", the user should be able to choose which time transform to display the keyframes in (as well as the software using a heuristic to determine which one automatically).