Create page.design property with values Material (default), Cupertino, Adaptive

[InesaFitsner]

Description
If page.design = ft.PageDesign.CUPERTINO CupertinoApp should be created.

If page.design = ft.PageDesign.ADAPTIVE CupertinoApp or MaterialApp should be created depending on page.platform value and adaptive property should be True for all child controls that don't have adaptive property set.

Code example to reproduce the issue:

import flet as ft


def main(page):

    page.appbar = ft.CupertinoAppBar(
        leading=ft.TextButton("Edit"),
        middle=ft.Text("Settings"),
        trailing=ft.IconButton(icon=ft.cupertino_icons.ADD),
    )

    page.window_width = 393
    page.window_height = 700
    page.theme_mode = ft.ThemeMode.LIGHT

    def dark_light_mode_changed(e):
        if page.theme_mode == ft.ThemeMode.LIGHT:
            page.theme_mode = ft.ThemeMode.DARK
        else:
            page.theme_mode = ft.ThemeMode.LIGHT
        page.update()

    dark_mode_checkbox = ft.Checkbox(
        value=False, label="Dark Mode", on_change=dark_light_mode_changed
    )

    page.add(
        dark_mode_checkbox,
    )


ft.app(target=main)