wandi: Rename the Update method of the Window interface to Display.
wandi: Make it easier to implement the Window interface by removing the Clear method from it.
wandiutil: Rename the Clearer interface to Filler.
wandiutil: Add Fill method to the Filler interface. The last commit missed to include it.
wandi: Remove unused import.
wandi: Replace the embedded Image interface of Window with dedicated Width and Height methods.