Deployment target is still 10.6

Allows for UI unit tests to be written:
- Ability to respond to expected message boxes
- Ability to fail test if unexpected message boxes occurs

Remove unused code

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!