Add support to create draggable window without title bar

### Description

The idea would be to add a function that does the same as a title bar, opening spaces for customizations,

I am currently working on a maui blazor app

I removed the default title bar, created one in blazor, but currently there is nothing that allows me to make the app draggable

In wpf there was a `DragMove()` function that did the service

Why doesn't maui have this kind of thing too?

This would make you have more customization possibilities and not get stuck in that windows title bar

### Public API Changes

An example of a title bar created in blazor

The `DragWindow()` function would be the function provided from maui

```csharp

@code {
private void MoveWindow() {
App.DragWindow();
}
}
```

### Intended Use-Case

I know maui is still new, but this kind of implementation would allow new creations without having to get stuck in the default title bar

And an app built in maui and blazor needs it

Start from the proposed App.DragWindow entry point and the Blazor MoveWindow handler shown in the issue. Review how MAUI exposes window and title-bar behavior on desktop platforms, then determine the supported platform scope and API shape. Done means a custom title-bar gesture can move a borderless window without relying on the default title bar.

Written by the indexing model from the issue text.