Switched each color-related interface's type from the str or String to Color (v4.0.0)

[simon-ritchie]

Since version 4.0.0, each color-related interface (for instance, fill_color, line_color, and so on) has been switched from the str or String to the newly added Color class.

Previous code example:

ap.Rectangle(
    x=50,
    y=50,
    width=50,
    height=50,
    fill_color="#00aaff",
)

New version code example:

ap.Rectangle(
    x=50,
    y=50,
    width=50,
    height=50,
    fill_color=ap.Color("#00aaff"),
)

This change increases type checkings' robustness (inspired by Flutter).

For more details, please see the following documents:

• Color class
• Colors class
• COLORLESS constant