Fix incorrect placement of the Settings Dialog on Multiple Monitors

The current monitor used was the 'default' one, the primary monitor
but if your default monitor was the laptop one, with the lid closed
like mine was for quite a bit, you'd spend like 20 minutes trying to
find the preferences dialog, like I did.
Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>

src/ui/SettingsDialog.cc

@@ -47,8 +47,11 @@ _ui(new Ui::SettingsDialog)
     _ui->setupUi(this);
 
     // Center the window on the screen.
+    QDesktopWidget *desktop = QApplication::desktop();
+    int screen = desktop->screenNumber(parent);
+
     QRect position = frameGeometry();
-    position.moveCenter(QApplication::desktop()->availableGeometry().center());
+    position.moveCenter(QApplication::desktop()->availableGeometry(screen).center());
     move(position.topLeft());
 
     QGCLinkConfiguration*  pLinkConf     = new QGCLinkConfiguration(this);