combine setActive and rideSelected

And pay attention to MainWindow::activeTab, such that only the active
tab redraws itself when changing rides.  This change really increases
GC's responsiveness when scrolling through the ride list.

src/PfPvWindow.cpp

@@ -88,6 +88,8 @@ PfPvWindow::PfPvWindow(MainWindow *mainWindow) :
 void
 PfPvWindow::rideSelected()
 {
+    if (mainWindow->activeTab() != this)
+        return;
     RideItem *ride = mainWindow->rideItem();
     if (!ride)
         return;