Multi Window Support

[Redth]

Description

This PR adds support for opening multiple windows on all the platforms that support them:

• Android
• iOS on iPad
• Mac Catalyst
• Windows

API Usage

The usage of the API is pretty simple:

var secondWindow = new Window {
    Page = new MySecondPage {
        // ...
    }
};

Application.Current.OpenWindow(secondWindow);

API Implementation

This API is implemented in the ApplicationHandler and uses a "OpenWindow" command with OpenWindowRequest arguments to hold state and launch options.

When a window is closed/backgrounded, there is a new Backgrounding event on Window that can be used to persist any string state to an object that the OS will preserve until it is time to resume that window. An example of state would be maybe a document path that the user has opened.

Then at some point in the future, when the user selects that window to resume, the state is provided via the IActivationState in CreateWindow. This state can the be read and used to re-load that document (if it is still available).

[mattleibow]