New version using QtQuick along with an experimental Map Background

Link ids were being used interchangeably with the mavlink channel for
the link. Link ids handed out as a numerically increasing integer. Once
the counter went past MAVLINK_COMM_NUM_BUFFERS all hell would break
loose and cause memory corruption.

Also removed partially implemented an incorrect getCommunicationStatus
api

Also many LinkManager API changes to further isolate all link
management inside LinkManager.

So this special casing is no longer needed

If it doesn’t exist it will be prefilled with all registered categories
set to false

Otherwise some messages say 'System MAV 0-20' and some say 'System 20'.

This signal passes the parameter type as well as param info

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.

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

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

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

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

Do not dynamic cast or de-reference QObject, since object is either in
destructor or may have already been destroyed.

Destroyed signal must have been working incorrectly in the past.

Make sure to no de-reference already deleted object

This removes double emit of signals.