v1: rework WindowDragArea

[ndonkoHenri]

Test Code

def main(page: ft.Page):
    page.add(
        ft.WindowDragArea(
            content=ft.Container(ft.Text("Test")),
            on_double_tap=print,
            on_drag_end=print,
            on_drag_start=print,
        )
    )

Summary by Sourcery

Rework the WindowDragArea control to use the new ConstrainedControl base in Python, consolidate its event properties, and provide a matching Dart-side implementation in the Flutter extension.

New Features:

• Add support for an on_drag_end event callback.
• Introduce a Dart WindowDragAreaControl widget to render and handle drag and double-tap behavior.
• Switch to using WindowEvent for double-tap maximize/unmaximize events and dedicated DragStartEvent/DragEndEvent types for drag callbacks.

Enhancements:

• Refactor Python WindowDragArea to declare properties (content, maximizable, event handlers) instead of subclassing GestureDetector.
• Update Flet core extension to register the new WindowDragArea control for Flutter integration.
• Remove custom handler methods and streamline event triggering logic.