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. The actual value of the plug remains the same, but the widget displays the CurrentFrame or FullRange when in those modes, and the PlaybackRange updates automatically in that mode (which does change the plug value).
Setting default value for CustomRange mode to demonstrate syntax.
PlaybackRange mode is read only as well.

I changed it's order in the list because I think it's more natural next to FullRange
Using metadata to store the actual CustomRange value.

This allowed PlaybackRange to set the plug value without losing the alternate value.
Merge pull request #1032 from andrewkaufman/dispatcherFramesUI

FrameRange UI displays the value that will actually be dispatched.