WIP issue #1173. Adding default suffix (extension). While at it, added selected filter support and removed unit test assert about its use.

QQuickWidget can’t handle this

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.

When using DeleteOnClose with a QQuickWidget, the shutdown sequence
causes the MainWindow destructor loop back onto itself.

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.

Allows parameter access from QML among other things

This is done by tweaking the application name when running unit test

This function is called automatically when a QSettings object is destroyed,
so is rarely needed to be called explicitly.

There are still some cases where
a QSettings object is passed-by-reference, so there I left the sync() calls as
it's unclear how permanent those objects are and if they'd call sync() before
application shutdown

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

Should have known better!

Subclass of QMessageBox which re-implements the static public
functions. There are two reasons for this:
1) The QMessageBox implementation on OSX does now show the title
string. This leads to message boxes which don't make much sense. So on
OSX we set title to text and text to informative text.
2) If parent is NULL, we set parent to MainWindow::instance. This
prevents message boxes which can go behind MainWindow

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

Clears the persisted settings for the QGC