Moving more things off of MainWindow

Moving methods from MainWindow to QGCApplication so that more things
can run without a window. Helpful for testing.
Move messagebox from MainWindow
Merge pull request #1148 from DonLakeFlyer/MainWindowReorg

Moving more things off of MainWindow
Move ui style from MainWindow to QGCApplication
Merge pull request #1149 from DonLakeFlyer/AppStyle

Move ui style from MainWindow to QGCApplication