This is in preparation for full Qml main window

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

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.