Fixed full-screen widgets all being moved to primary screen on OSX

(due to commit 9adc899).

Fullscreening should now work as expected in OSX 10.9 and 10.10, with
one or multiple monitors.

src/frameworks/UBPlatformUtils_mac.mm

@@ -598,14 +598,14 @@ void UBPlatformUtils::showFullScreen(QWidget *pWidget)
     
     pWidget->showMaximized();
 
-    /* Hack. On OS X 10.10, showMaximize() resizes the widget to full screen;
-    but on 10.9, it is placed in the "available" screen area (i.e the screen area minus
-    the menu bar and dock area). So we have to manually resize it to the total screen height,
-    and move it up to the top of the screen (0,0 position) */
+    /* Bit of a hack. On OS X 10.10, showMaximized() resizes the widget to full screen (if the dock and
+     * menu bar are hidden); but on 10.9, it is placed in the "available" screen area (i.e the
+     * screen area minus the menu bar and dock area). So we have to manually resize it to the
+     * total screen height, and move it up to the top of the screen (y=0 position). */
 
     QDesktopWidget * desktop = QApplication::desktop();
     pWidget->resize(pWidget->width(), desktop->screenGeometry().height());
-    pWidget->move(0, 0);
+    pWidget->move(pWidget->pos().x(), 0);
 
 }