Refactor colour picker

[billyquith]

• It get controls by name!
• It is slow to draw the colour spectrum display as it draws each individual pixel as a rectangle, resulting in needing CTT (see #27). This colour be replaced with render to pixel buffer and then blit that somehow (new feature).

[billyquith]

@sfinktah I should be able to whip you up an actual "HSV" color picker too, I've already had to replicate imgui's conversion routines for HSV and RGB conversion. But hell if I know what I'd call the class. Also, my implementation used integers, not floats, so the range for a HSV value would be [360, 100, 100, 255] (yeah, it doesn't really make a lot of sense, i know) :p

Perhaps there needs to be some color conversion routines. Then each implementation of the color pickers can use these routines.