Now works for more than Serial links

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

This prevents mavlink signals from being emitted after link is
disconnected

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.

So no need for connect window close test here

This removes confusion as to whether you call UAS::requestParameters
(incorrect) or QGCUASParamManager::requestParameterList (correct) from
client code.

This allows use of compile time Object::connect syntax

Simple starting point to view testing

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

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