Fix recent regression in IA2 live regions support where certain text was being announced even if aria-relevant was only additions.
Specifically:
* getTextFromIAccessible: add an includeTopLevelText argument (true by default) which controls whether text that is not an embedded object is included in the text collected.
* all calls to getTextFromIAccessible that use default argument values: don't specify the values.
* When adding text due to a text inserted or text updated event, only tell getTextFromIAccessible to include top level text if aria-relevant allows text.