Refactoring on MainWindow's view and settings code

Main change is removing the default view from the MainWindow. It didn't need to exist so long as the cases where a view isn't set are handled correctly.

Additionally I ran into some weirdness with settings and how they're saved on shutdown and loaded at startup, so I fixed that since I was there. Specifically, several settings were never saved and reloaded with the old code: low-power mode, the theme, window position, custom mode, the view, and the UAS-connected view. 

Only contentious commit might be a56bbca, but I don't think the PFD should be in the Planner view. It never existed there before as the code was wrong, but that was fixed in the previous commit (5262217).

Resolves #1044, #1017, #1013.