Create GuiUtils functions createColorIcon() and createOpacityIcon()

* createColorIcon creates a QIcon which visualizes the given colors
  using rectangles. A background icon can be provided, in that case
  only the lowest 25% of the icon will be filled by the rectangles.
* createOpacityIcon creates a QIcon that visualizes a given opacity
  using the current foreground color and a checkerboard background.

These functions are now used in place of colorIcon, colorPicerIcon,
opacityIcon in AnnotationActionHandler. The new functions have some
advantages: support most common icon sizes, and dark color schemes.