Return the index of the selected tab item, or -1 if there
is no selection.
@return the index of the selected tab item or -1
@exception SWTException <ul>
<li>ERROR_THREAD_INVALID_ACCESS when called from the wrong thread</li>
<li>ERROR_WIDGET_DISPOSED when the widget has been disposed</li>
</ul>
Return the index of the selected tab item, or -1 if there is no selection.
@return the index of the selected tab item or -1
@exception SWTException <ul> <li>ERROR_THREAD_INVALID_ACCESS when called from the wrong thread</li> <li>ERROR_WIDGET_DISPOSED when the widget has been disposed</li> </ul>