No longer always fall back to using the MSAA childID and accChildCount as position...

No longer always fall back to using the MSAA childID and accChildCount as position information. In many cases this information was always wrong (e.g. incorrect position information for menu items, task bar buttons etc). Specific changes:
*IAccessible NVDAObject: add a 'allowIAccessibleChildIDAndChildCountForPositionInfo' class variable, which the positionInfo property now checks. If true then it will fall back to using childID and accchildCount, if there is no richer IAccessible2 info available. This class variable is set to False by default.
*TaskListIcon IAccessible NVDAObject: set allowIAccessibleChildIDAndChildCountForPositionInfo to True as position info from MSAA is always correct, and quite useful gere,
*SysListView32ListItem IAccessible NVDAObject: rewrite the positionInfo property so that it does not rely on the super implementation, but instead specifically grabs what it needs.