Link manager unit test and fixes

Fix linux compiler warning

Largest change is that once a link is added to LinkManager it maintains
ownership and is responsible for delete. If you need to delete a link
use LinkManager::delete. Also added guard code to assert if a Link is
deleted outside of LinkManager.

Allows you to run a single unit test from the command line

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