Add 'New color scheme' functionality

[lobster-mane]

The pull request adds new function that allows users to add new custom color schemes. This is achieved through a new function called do_new_scheme that takes a name and three colors as hex values for inputs, checks if the color scheme already exists, and if not, creates it by copying the blue folder icons and replacing the color hex values.

I'm personally using this to automatically change folder colors based on my wallpaper using matugen and it works great.

Additional helper functions:

• `_is_valid_hex()' -> validates a single hex color value
• validate_hexes() -> validates all colors from user input