feat: wrap Views into a background container

[ndonkoHenri]

Description

Closes #3743

Test Code

import flet as ft


def main(page: ft.Page):
    page.horizontal_alignment = ft.CrossAxisAlignment.CENTER
    page.vertical_alignment = ft.MainAxisAlignment.CENTER
    
    page.bgcolor = ft.colors.TRANSPARENT
    page.decoration = ft.BoxDecoration(
        gradient=ft.LinearGradient(
            colors=[ft.colors.RED, ft.colors.BLUE],
            begin=ft.alignment.top_left,
            end=ft.alignment.bottom_right,
            stops=[0.0, 1.0],
        ),
        image=ft.DecorationImage(
            src="https://images.unsplash.com/photo-1547721064-da6cfb341d50",
            fit=ft.ImageFit.COVER,
            opacity=0.2,
        ),
    )

    page.add(ft.Text("Hello, World!", size=55),)


ft.app(main)

Screenshots (if applicable):

Summary by Sourcery

Add support for background and foreground decorations in View and Container components, enabling more flexible UI designs. Enhance border radius parsing with default values and update macOS client code to use modern Swift attributes.

New Features:

• Introduce background and foreground decoration properties to the View and Container components, allowing for more customizable UI elements.

Enhancements:

• Enhance the borderRadiusFromJSON function to accept a default value, improving flexibility in border radius parsing.
• Update the AppDelegate in the macOS client to use the @main attribute, modernizing the Swift code.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request introduces a new feature to wrap Views into a background container. The changes primarily affect the View and Page classes in the Python SDK, as well as corresponding Dart implementations. The main additions include new properties for decoration and foreground_decoration, which allow for more customization of the background and foreground of views and pages.

File-Level Changes

Files	Changes
sdk/python/packages/flet-core/src/flet_core/view.py	Added decoration and foreground_decoration properties to the View class
sdk/python/packages/flet-core/src/flet_core/page.py	Added decoration and foreground_decoration properties to the Page class
packages/flet/lib/src/controls/page.dart	Implemented background and foreground decoration parsing and application in the Dart ViewControl class
packages/flet/lib/src/utils/borders.dart	Modified borderRadiusFromJSON function to accept a default value
sdk/python/packages/flet-core/src/flet_core/container.py packages/flet/lib/src/controls/container.dart	Added foreground_decoration property to the Container class
packages/flet/lib/src/utils/theme.dart	Updated parseTabBarTheme function to use the modified borderRadiusFromJSON
packages/flet/lib/src/controls/canvas.dart	Modified FletCustomPainter class to use the updated borderRadiusFromJSON function
client/macos/Runner/AppDelegate.swift	Updated MacOS AppDelegate annotation from @NSApplicationMain to @main

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[sourcery-ai]

Hey @ndonkoHenri - I've reviewed your changes - here's some feedback:

Overall Comments:

• The change in client/macos/Runner/AppDelegate.swift from @NSApplicationMain to @main seems unrelated to the main purpose of this PR. Could you explain the reasoning behind this change?

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟡 Complexity: 1 issue found
• 🟢 Documentation: all looks good

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[ndonkoHenri]

The change in client/macos/Runner/AppDelegate.swift from @NSApplicationMain to @main seems unrelated to the main purpose of this PR. Could you explain the reasoning behind this change?

Yes sir! 🙂
Running the above test code in my dev-environment, Flutter notified me (see below) that NSApplicationMain in AppDelegate.swift was deprecated, and it automatically made the appropriate modification.

macos/Runner/AppDelegate.swift uses the deprecated @NSApplicationMain attribute, updating.