Instead us a Toolbox concept which hangs off of QGCApplication

The current monitor used was the 'default' one, the primary monitor
but if your default monitor was the laptop one, with the lid closed
like mine was for quite a bit, you'd spend like 20 minutes trying to
find the preferences dialog, like I did.
Signed-off-by: Tomaz Canabrava <tomaz.canabrava@intel.com>

The goal of this first installment is to organize the code a bit so it's more readable,
clean up a bit of left over cruft, and manage link configurations (and links in general).

Now works for more than Serial links

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.

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

Using QFileDialog directly on OS X can hang intermittently due to Qt
bug.

Also renamed from QGCSettingsWidget to SettingsDialog to better reflect
usage.

No major functionality changes.

Selecting the desired stylesheet now works correctly, including saving settings between application launches.

Reworked the styling UI. Now there are 4 options: light, dark, custom_light, and custom_dark. Things don't quite work right, but the proof-of-concept is done.