[UI] Color eyedropper (pipette)

[JulesFouchy]

We want to add an eyedropper to the color picker, so that it is able to copy a color from the pixel under the mouse. It should work anywhere on the screen, even outside of Coollab's window.
There might be a glfw API to do this, or another cross-platform library. Worst case we will have to write this ourselves for all OS.