Fixes an issue by which the PFD was continuing to process events even when it was no longer visible.

1. Plug in board
2. Start GCS
3. On Welcome screen, processor load is 6%
4. Go to Flight screen
5. Processor load increases to 20%
6. Return to Welcome screen, processor load stays at 20%

This is because the QML is still processing the state changes, even though it is no longer visible.

On OSX with Qt 4.8, this saved 14% processor load. This might not be an issue on other platforms.