Also only update the full tree one time once the full parameter list is
ready.

Previously link were just destroyed

This makes unit test output easier to read

Previously link were just destroyed

This makes unit test output easier to read

Renamed some variables.
Removed unused joystick settings stuff.
Deleted unused actions.

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

Fixes bugs with savings settings for the window state, theme, UAS-connected view, and low-power mode.
Also removes some unneccesary logic that attempted to handled unclean shutdowns, which I haven't
been able to reproduce.

It was a poor proxy for better coding around the problem of bad settings or the lack of
settings on first-run.

No functionality changes in this code, just code clarify improvements.

Splash screen is optional. Also remove some unused class definition.

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