- FirmwarePlugin is the new place for code which is specific to
firmware flight stack
- FirmwarePlugin currently supports flight mode apis
- Modify code to use new FirmwarePlugin support

Plus a large number of other changes to allow for orderly shutdown of
objects without crashes or asserts. This is need for unit tests to
create/delete global state around every test.

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

Used for unit testing