RGB color slider instead of the limited color set

[Zajc]

As in the title, we really should be able to paint the blocks in any given color, instead of having to use very limited palette. Especially that some colors like most very dark colors or shades of green are VERY limited. Isuggest using sliders of red, green and blue as well as opacity of the paint in the color selection menu.

[Rinart73]

Or to add the color wheel that is already used in the Settings -> UI color