add thumb_icon to Switch

[ndonkoHenri]

Close #1638

Example:

import flet as ft


def main(page: ft.Page):
    page.window_height, page.window_width = 160, 200

    page.add(
        ft.Switch(
            thumb_color={
                ft.MaterialState.HOVERED: ft.colors.GREEN,
                ft.MaterialState.FOCUSED: ft.colors.RED,
                ft.MaterialState.DEFAULT: ft.colors.BLUE,
            },
            thumb_icon={
                ft.MaterialState.SELECTED: ft.icons.CHECK,
                ft.MaterialState.DEFAULT: ft.icons.CLOSE,
            },
        )
    )


ft.app(target=main)

Capture: