`Rive` Control

[ndonkoHenri]

Closes #89

Note: The implementation of on_init in Rive control causes it to loose animation.

Test Code:

import flet as ft


def main(page):
    page.window_always_on_top = True
    page.horizontal_alignment = ft.CrossAxisAlignment.CENTER
    page.vertical_alignment = ft.MainAxisAlignment.CENTER

    page.add(
        ft.Rive(
            "https://cdn.rive.app/animations/vehicles.riv",
            placeholder=ft.ProgressRing(),
            on_init=lambda e: print("Rive control initialized!"),
            width=300,
            height=200,
        )
    )


ft.app(target=main)