feat: colors convert from hex

[kkharji]

Closes #1492

[kkharji]

fix: resulting colors are incorrect

[kkharji]

@emilk 🤔 that's odd one test fail with

thread 'color::test_to_hex_to_rgba' panicked at 'assertion failed: `(left == right)`
  left: `Color32([20, 1, 7, 2])`,
 right: `Color32([245, 40, 145, 255])`', epaint/src/color.rs:587:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I explicitly set rgba to Color32::from_rgba_unmultiplied(245, 40, 145, 2); but it produced [20, 1, 7, 2]

btw 2 because I wanted 0.2 not sure if this is correct

[emilk]

Suggested change

	let rgba = Color32::from_rgba_unmultiplied(245, 40, 145, 2);
	let hex = Color32::from_hex("#f5289133").unwrap();
	assert_eq!(rgba, hex);
	let rgba = Color32::from_rgba_unmultiplied(0xf5, 0x28, 0x91, 0x33);
	let hex = Color32::from_hex("#f5289133").unwrap();
	assert_eq!(rgba, hex);

[emilk]

The benefit with a macro (e.g. egui::color!(#f5289133)) is that errors can be caught at compilation time. If users want to have dynamic string they should probably be using one of the many css color parsing crates.

[emilk]

Closed by #1596