Add support for multiple displays in DeviceDisplay

[mattleibow]

Description

For desktop applications, there might be a need to have more information on the current screen the window is on or know about all the screens in order to move the window to a different display.

I believe all the platforms support multiple screens:

• Windows and macOS both support adding new physical monitors/projectors
• iOS may allow extending to the mac or iPad or something?
• Android devices can have monitors and/or multiple screens on some phones.

Currently, the DeviceDisplay API only has info on the "main" display and this usually is fine for mobile. But, for desktop apps the main display may not actually be the current display.

public interface IDeviceDisplay
{
	DisplayInfo MainDisplayInfo { get; }
	event EventHandler<DisplayInfoChangedEventArgs> MainDisplayInfoChanged;
}

Public API Changes

A new API would be to add a display id or instance to Window objects, or at the very least add new ways to get more information about what monitors are in use.

public interface IDeviceDisplay
{
	// Gets the display information for the specified index
	DisplayInfo GetDisplay(int index);

	// OR: Gets the display information for the specified index
	DisplayInfo GetDisplay(DisplayId id);

	// Gets the number of displays attached to the device
	int DisplayCount { get; }

	// OR Gets the number of displays attached to the device
	DisplayId[] Displays { get; }

	// And event that gets raised if any display changes or a new one is added (or removed)
	event EventHandler<DisplayInfoChangedEventArgs> DisplayInfoChanged;
}

public class DisplayInfoChangedEventArgs
{
	// The index of the display that is changing
	public int Index { get; }

	// OR: The ID of the display that is changing
	public DisplayId Id { get; }
}

public struct DisplayId
{
	// something that will help MAUI identify different displays
	internal <platform-type> PlatformId { get; }
}

public class Window
{
	// Get the index of the display in the DeviceDisplay.GetDisplay list
	public int DisplayIndex { get; }

	// OR: Get the display ID
	public DisplayId DisplayId { get; }
}

Intended Use-Case

When launching an app, there may be a desire to launch on a specific monitor. Being able to find out where the monitors are so windows can be moved programmatically.

Potentially Related Issues

• DeviceDisplay.Current.MainDisplayInfo not working #12214
• [Mac Catalyst] DeviceDisplay.MainDisplayInfo is not refreshed #22642
• Multi-Window in Windows with all options #6210
• [Mac Catalyst] DeviceDisplay.MainDisplayInfoChanged event not getting raised #22634
• [Window] Choose the screen that the window will open #13777