Use splitter's section size to determine whether scanner pane is visi…

…ble.

This was a regression in e8ea2b3, where the visibility predicate
was changed to something more readable. As it turned out, the visibility
of the widget does not indicate whether it is visible: the splitter may
be collapsed.

The predicate has now been reverted to what it was previously.

Test plan:

- Launch DME.
- Collapse the scanner pane.
- Exit DME.
- Launch DME.

Without this change, the **View > Scanner** would be checked, even
though the pane is collapsed.

Source/GUI/MainWindow.cpp

@@ -86,7 +86,7 @@ void MainWindow::makeMenus()
   m_actScanner->setCheckable(true);
   QTimer::singleShot(0, [this]() {
     QSignalBlocker signalBlocker(m_actScanner);
-    m_actScanner->setChecked(m_scanner->isVisible());
+    m_actScanner->setChecked(m_splitter->sizes()[0] > 0);
   });
 
   m_actQuit = new QAction(tr("&Quit"), this);