text selection isn't accessible on the mac

When selecting text, whether in a QLineEdit, QTextEdit, or editable
QComboBox, when the Mac's accessibility framework VoiceOver is turned
on, the text should be read outloud. So if the word "foo" was highlighted, it would read "foo highlighted".

With a Qt application, this doesn't happen. Nothing happens when
selecting text in Qt.

This appears to be because in qaccessible_mac.cpp, there isn't support
for the kAXSelectedTextRangeAttribute and kAXSelectedTextAttribute
attributes.

These two attributes are documented as required attributes here: http://developer.apple.com/documentation/Accessibility/Reference/AccessibilityCarbonRef/Reference/reference.html