No functionality changes in this code, just code clarify improvements.

This keeps TeamCity from hanging on a dialog box

This was leftover from a problem I was having with virtual slots

Splash screen is optional. Also remove some unused class definition.

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

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

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