Add setValue function to inline editor widget.

This change adds a utility function to set the value of an inline editor,
regardless of whether the editor is open or not.

Testing Done:
Used this in conjunction with another pending change to add reviewer
suggestions. Opened a review request, clicked on a reviewer suggestion (which
triggers a call to appendToFieldValue, ensured that the suggestion was filled
into the inline editor. Hit OK and ensured that it was saved in the editor. Did
the same with cancel to ensure that it was not remembered.

Reviewed at https://reviews.reviewboard.org/r/5030/