AppBar.system_overlay_style prop

[ndonkoHenri]

Closes #2573
Test Code:

import flet as ft


def main(page: ft.Page):

    page.appbar = ft.AppBar(
        leading=ft.Icon(ft.icons.PALETTE),
        leading_width=40,
        title=ft.Text("AppBar Example"),
        center_title=False,
        bgcolor=ft.colors.SURFACE_VARIANT,
        system_overlay_style=ft.SystemOverlayStyle(
            status_bar_color=ft.colors.YELLOW,
            system_navigation_bar_color=ft.colors.GREEN,
            system_navigation_bar_divider_color=ft.colors.BLUE_ACCENT,
            enforce_system_navigation_bar_contrast=True,
            enforce_system_status_bar_contrast=True,
        ),
    )
    page.add(ft.Text("Body!"))


ft.app(target=main)