Revert premature assumption that adding support for selected filters in Open Save would not affect all other file dialog operations. As is, unit tests requires all or nothing.

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

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