Add RGB Hexadecimal text input to colour picker

[miccou]

• Adds a text input above the colour picker.
• If you drag the mouse to pick a colour, the hex code updates.
• Hex code inputs is restricted to base16 chars.
• Pressing enter after typing or pasting a hex colour code chooses the same colour by hue and sat but with 100% luminance.
• Pressing enter without a valid 3 or 6 character hex colour code will do nothing
• Clicking out of the colour picker will reset the text box to match the selected colour

[miccou]

@olivia would yourself or another maintainer have time to consider this PR?

[miccou]

Thanks for your feedback @olivia, I've addressed these with commits to my feature branch.