[Spec] Implement Device class functionality to Core

[jsuarezruiz]

[The feature]

The Device class contains a number of properties and methods to help developers customize layout and functionality on a per-platform basis.

• Flags - Used to use experimental APIs in Xamarin.Forms. Obsolete.
• SetFlags - Remove.
• FlowDirection -Move to Window.
• SetFlowDirection - Remove.
• Idiom - Move to MauiContext and make it use essentials.
• Info - Obsolete.
• SetIdiom - Remove.
• SetTargetIdiom - Remove.
• IsInvokeRequired - obsolete (hidden)
• PlatformInvalidator - Try to remove.
• Invalidate - Try to remove.
• RuntimePlatform - Make it use essentials and move these to Window, use Application.Windows.First() to back these.
• BeginInvokeOnMainThread(Action)
• GetNamedColor(string) - Move to .NET MAUI Graphics.
• GetNamedSize (multiple overloads) - TODO find the 'current' way to do this.
• StartTimer - Move to Application or Window? Maybe this goes away and is handled by animation stuffPlatformServices
• RequestedTheme - Move to Window, use Essentials implementation.
• RuntimePlatform - Remove.
• GetHash - Remove.
• GetMD5Hash - Remove.
• GetNamedColor - Remove.
• GetNamedSize - Remove.
• GetNativeSize - Gets renderer or handler and calls GetDesiredSize - remove.
• GetStreamAsync - Remove.
• GetUserStoreForApplication - Remove.
• OpenUriAction - Obsolete, use essentials.
• QuitApplication - Move to Application.
• StartTimer - Move to window? Maybe this goes away and is handled by animation stuff
• Dispatcher - find window of mainpage and use that dispatcher.
• MainPage / CreateWindowMake CreateWindow return MainPage, if no mainpage is set throwSet MainPage, go through windows to find window with current mainpage, and replace mainpage.

Difficulty : [medium]

[mattleibow]

Duplicate of #1965