Returns <code>true</code> if the receiver is selected,
and false otherwise.
<p>
When the receiver is of type <code>CHECK</code> or <code>RADIO</code>,
it is selected when it is checked. When it is of type <code>TOGGLE</code>,
it is selected when it is pushed in. If the receiver is of any other type,
this method returns false.
@return the selection state
@exception SWTException <ul>
<li>ERROR_WIDGET_DISPOSED - if the receiver has been disposed</li>
<li>ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the receiver</li>
</ul>
Returns <code>true</code> if the receiver is selected, and false otherwise. <p> When the receiver is of type <code>CHECK</code> or <code>RADIO</code>, it is selected when it is checked. When it is of type <code>TOGGLE</code>, it is selected when it is pushed in. If the receiver is of any other type, this method returns false.
@return the selection state
@exception SWTException <ul> <li>ERROR_WIDGET_DISPOSED - if the receiver has been disposed</li> <li>ERROR_THREAD_INVALID_ACCESS - if not called from the thread that created the receiver</li> </ul>