add barrier color as an option to showDialog function and python class

[OwenMcDonnell]

Description

The showDialog function in flutter accepts a barrierColor parameter to change (or make transparent) the modal barrier color. Addition of this option to Flet's AlertDialog resolves an issue mentioned in #4130.

Fixes #4130

Test Code

import flet as ft


def main(page: ft.Page):
    page.window.width = 450
    page.window.height = 450
    page.window.bgcolor = ft.colors.TRANSPARENT
    page.bgcolor = ft.colors.TRANSPARENT
    page.window.frameless = True
    page.window.center()

    def close_dialog(e):
        page.close(alert)
        page.update()

    alert = ft.AlertDialog(
        modal=True,
        title=ft.Text('Alert Dialog'),
        content=ft.Text('This is an alert dialog'),
        actions=[
            ft.TextButton('OK', on_click=close_dialog),
        ],
        shadow_color=ft.colors.TRANSPARENT,
        surface_tint_color=ft.colors.TRANSPARENT,
        barrier_color=ft.colors.TRANSPARENT
    )
    
    def open_dialog(e):


        page.open(alert)
        page.update()
    

    
    def close_app(e):
        page.window.close()
    
    page.add(
        ft.WindowDragArea(content=ft.Container(
                content=ft.Column(
                    controls=[
                        ft.ElevatedButton('Open dialog', on_click=open_dialog),
                        ft.ElevatedButton('Close app', on_click=close_app),
                        ],
                    alignment=ft.MainAxisAlignment.CENTER,
                    spacing=10,
                    ),
                bgcolor=ft.colors.SURFACE_CONTAINER_HIGHEST,
                border_radius=30,
                alignment=ft.alignment.center,
            ),
            expand=True
        )

    )


ft.app(target=main)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Checklist:

• I signed the CLA.

• My code follows the style guidelines of this project

• I have performed a self-review of my own code

• My changes generate no new warnings

• I have made corresponding changes to the documentation (if applicable)

Screenshots (if applicable):

Additional details

[ndonkoHenri]

I guess we can add this prop to showDialog in CupertinoAlertDialog, DatePicker and TimePicker too?

It's been fixed