FrameRange UI displays the value that will actually be dispatched.

This overrides the widget text on the `Dispatcher` frameRange plug to show the value that will be dispatched, even when not in `CustomRange` mode. In `CurrentFrame` and `FullRange` modes, the actual value of the frameRange plug remains unchanged, but the widget displays the `CurrentFrame` or `FullRange`. In `PlaybackRange` mode, the frameRange plug value updates with changes to the playback range.

I just came up with this as @johnhaddon merged the other pull request. I think this sort of change will resolve Yuta's confusion about what range he is disptaching, though I realize John might not like it because it masks the real plug value... what do you think? Does the clarity to the user outweigh the confusion to the programmer?