Add read/write alt_text property to BaseShape for accessibility support. Includes implementation, documentation, and tests for the new property.

Signed-off-by: Huan Di <huandzh@gmail.com>

Add deleter for alt_text property to allow complete removal of alternative text
from shapes. Update getter to return None instead of empty string when no alt
text is present. Add BDD tests for alt_text get/set/delete operations.

Signed-off-by: Huan Di <huandzh@gmail.com>

Add test cases to verify proper deletion of alt text attribute from various shape types including Shape, BaseShape, GraphicFrame, and Picture. Includes fixture with multiple test scenarios.

Signed-off-by: Huan Di <huandzh@gmail.com>

Merged Changes: Implement alternative text property for shapes to support screen readers. Includes getter, setter, and deleter methods, along with documentation and comprehensive tests.

…uced when merging master.

Signed-off-by: huandzh <huandzh@gmail.com>