Create a MainWindow + new titlebar

[niels9001]

We currently create a new Window upon launch, and settings the NavigationRootPage as its content.

Creating a MainWindow and moving that code there makes things easier when we want to make use of certain Windowing APIs and the new TitleBar.

Once the new TitleBar control is in stable, we can move the NavigationView-button and searchbox to the titlebar area per latest design guidance:

[Jay-o-Way]

Must solve #1605

[Jay-o-Way]

Also note (for example) #145 which seems to be a work-around that probably should be reverted. If you create the MainWindow with the Navigation from scratch, you should find this. FYI @marcelwgn right?

[Jay-o-Way]

Has anybody tested this on Windows 10? Thinking of possible issues like microsoft/microsoft-ui-xaml#10489