Returns <code>true</code> if the receiver's header is visible,
and <code>false</code> otherwise.
<p>
If one of the receiver's ancestors is not visible or some
other condition makes the receiver not visible, this method
may still indicate that it is considered visible even though
it may not actually be showing.
</p>
@return the receiver's header's visibility 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's header is visible, and <code>false</code> otherwise. <p> If one of the receiver's ancestors is not visible or some other condition makes the receiver not visible, this method may still indicate that it is considered visible even though it may not actually be showing. </p>
@return the receiver's header's visibility 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>