Singletons tracked by QGCApplication

- allow unit tests to use QGCApplication
- clear global singletons for each unit test run

File renames

Be careful with QObject:::destroyed signal

QGCMessageBox implementation

New File Locations key not set in QSettings at boot

Should have known better!

Subclass of QMessageBox which re-implements the static public
functions. There are two reasons for this:
1) The QMessageBox implementation on OSX does now show the title
string. This leads to message boxes which don't make much sense. So on
OSX we set title to text and text to informative text.
2) If parent is NULL, we set parent to MainWindow::instance. This
prevents message boxes which can go behind MainWindow

Do not dynamic cast or de-reference QObject, since object is either in
destructor or may have already been destroyed.

New mavlink logging support

Plus change to use QGCFileDialog

Using QFileDialog directly on OS X can hang intermittently due to Qt
bug. QGCFileDialog works around this by not using the native dialogs.

Using QFileDialog directly on OS X can hang intermittently due to Qt
bug.

Remove PIXHAWK specific board support

Destroyed signal must have been working incorrectly in the past.

- Logging now always on to temp file
- User prompted to save temp file after disconnect
- Orphaned temp logs checked for on boot
- Logs saved to new save file location
- Log file starts/stops with first and last connnection

- Also renamed QGCCore->QGCApplication to better reflect usage.
- New global qgcApp api to access main QHCApplication singleton

Make sure to no de-reference already deleted object

Also renamed from QGCSettingsWidget to SettingsDialog to better reflect
usage.

This avoids collision with main app SettingsDialog class