Cocoa: Use the whole geometry when in fullscreen mode for popup menus

When in full screen, then the geometry available is the whole screen
not including the menubar or dock area. This ensures that it does
not leave space for the dock area.

Task-number: QTBUG-39403
Change-Id: I15d7b7764d83282e1ee057628680a338b72711a6
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -347,6 +347,8 @@ QVariant QCocoaTheme::themeHint(ThemeHint hint) const
         return QVariant(int(HoverEffect));
     case QPlatformTheme::SpellCheckUnderlineStyle:
         return QVariant(int(QTextCharFormat::DotLine));
+    case QPlatformTheme::UseFullScreenForPopupMenu:
+        return QVariant(bool([[NSApplication sharedApplication] presentationOptions] & NSApplicationPresentationFullScreen));
     default:
         break;
     }