New fpgApplication.GetScreenPixelColor() for X11

We can now color pick a value on a X11 desktop.
colorwheel demo: Hex label now has the normal naming convention.
colorwheel demo: now has a color picker button

* Click and hold the mouse. Then move the mouse and release. The
  colorwheel will then update the color under the mouse cursor.
* Enable the "Continuous" checkbox and do the same as above. As you
  move the mouse, the colorwheel will continuously update.

NOTE:
  The internal functionality used is still only for X11.