Made event.change evaluate to whole incoming text.

Currently event.change evaluates from first point of difference to end of the string. This MR modifies it to evaluate to whole incoming text. Now we evaluate whatever change is coming in, even if that change contains some common substring with original string (not considering only from the first point of difference).

The current definition makes it difficult to evaluate the final value from event.value and event.change within Javascript. This change is also better from compatibility POV as other readers such as Adobe and PDF.js make similar calculations.

Dependent on MR981.