Fix SnackBar action handling, add examples and tests

[FeodorFitsner]

Introduces a new Python example demonstrating SnackBar actions, updates documentation with new examples and images, and enhances integration tests for SnackBar actions. The Dart SnackBar control is updated to better distinguish between string and control-based actions. Minor doc corrections and test refactoring are also included.

Summary by Sourcery

Improve SnackBar action handling by fixing the Dart control logic, add a new Python example for simple and custom actions, update documentation with examples and images, and introduce comprehensive integration tests.

New Features:

• Add a Python example script demonstrating simple and custom SnackBar actions

Bug Fixes:

• Fix Dart SnackBar control to correctly distinguish and handle string-based and control-based actions and trigger action events properly

Enhancements:

• Refactor Python SnackBar tests by renaming the basic test, adjusting page sizes, and centralizing screenshot handling

Documentation:

• Update snack_bar documentation with new action examples and images
• Correct typos and references in SnackBarAction and ColorScheme docstrings

Tests:

• Add Python integration tests for simple and custom SnackBar actions in material controls and examples

[sourcery-ai]

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