Add basic single-word selection to CaptionTextBox

Adds single-word selection to CaptionTextBox. Single words can be
highlighted by moving left and right with the arrow keys.
Remove get-index methods from CaptionWordList
Fix Word Selection Issue in CaptionTextBox

Due to issues with selecting single letter words (such as "a" or "I"),
CaptionTextBox now selects the word and a trailing space character.