AlertDialog - icon, bgcolor, elevation

[ndonkoHenri]

Closes #2520

Test Code:

import flet as ft

def main(page: ft.Page):

    dlg_modal = ft.AlertDialog(
        modal=True,
        title=ft.Text("Please confirm"),
        content=ft.Text("Do you really want to delete all those files?"),
        icon=ft.Icon(ft.icons.WARNING_OUTLINED),
        bgcolor=ft.colors.RED_200,
        elevation=5,
        actions=[
            ft.TextButton("Yes", on_click=lambda e: page.close_dialog()),
            ft.TextButton("No", on_click=lambda e: page.close_dialog()),
        ],
        actions_alignment=ft.MainAxisAlignment.END,
        on_dismiss=lambda e: print("Modal dialog dismissed!"),
    )

    page.add(
        ft.ElevatedButton("Open modal dialog", on_click=lambda e: page.show_dialog(dlg_modal)),
    )

ft.app(target=main)