Moving methods from MainWindow to QGCApplication so that more things
can run without a window. Helpful for testing.

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

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

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

Switched from flite for speech on the Mac to built-in MacOSX speech synthesizer. Fixed build system for 64-bit builds (using official SDL.framework, and Qt frameworks).