StyledText.handleTextChanging

Updates the screen to reflect a pending content change.

@param event .start the start offset of the change @param event .newText text that is going to be inserted or empty String if no text will be inserted @param event .replaceCharCount length of text that is going to be replaced @param event .newCharCount length of text that is going to be inserted @param event .replaceLineCount number of lines that are going to be replaced @param event .newLineCount number of new lines that are going to be inserted