Provide a `Window`-less mode of application builder

[MartinZikmund]

What would you like to be added:

Currently the application host always also creates a Window. This is quite impractical, as the user may want to create the window on their own (maybe use a custom class derived from Window, which is possible in WinUI and Uno.WinUI. In addition, in multiwindow apps the "initial" window may not even be the "primary", so it does not make much sense for extensions to single it out this way.

In addition getting the window currently means one needs to take the builder itself, so it means something like this is needed:

var builder = this.CreateBuilder(args)
	.Stuff()
	.Things();

var myWindow = builder.Window;

var host = builder.Build();

Instead of:

var host =  this.CreateBuilder(args)
	.Stuff()
	.Things()
	.Build();

// I have no way to get the Window now...

Why is this needed:

Usability in multiwindowed and multi-instance apps

For which Platform:

All

Anything else we need to know?