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.

The proposed fix disables the UAVO connection to the QML when the QML is hidden. On OSX with Qt 4.8, this saved 14% processor load. This might not be an issue on other platforms. It also might be fixed in Qt 5.1: https://bugreports.qt-project.org/browse/QTBUG-29137, but this is unclear since the described behavior is not the same as here.

UPDATE: On all tested platforms, this shows sizable gains, although it's interesting to note that on Linux platforms there is variability on whether the gain comes as a result of switching to the Welcome screen or minimizing/hiding the GCS. It's unclear why QML doesn't behave universally across all platforms, but this PR does not seek to address that.