Add GetCaptionWordIndex method to CaptionWordList

Adds GetCaptionWordIndex, which gets the index of a caption word from
the list given a string index.
Add GetCaptionWordIndexTest

Adds a unit test to ensure proper functionality of the GetCaptionWord
method. Adds corrections to GetCaptionWord method to ensure it passes
the test.
Change GetCaptionWordIndexAt method

Changes GetCaptionWordIndexAt method so that it does not throw an
ArgumentOutOfRangeException when the given stringIndex is too big.
Updates the related unit test to adhere to this and to be more
thourough.
Add GetStringStartIndexAt Method

Adds the GetStringStartIndexAt method to CaptionWordList. Adds a unit
test to ensure correct execution of method.
Attempt OnSelectionChanged Progress

Code does not work at the moment. Progress commit so that it can be
returned to later.
Add BeginIndex and EndIndex to CaptionWord