Custom pointers for drag and drop operations

This pull request builds on top of the plug drag and drop request, adding custom pointers for all relevant drag and drop operations throughout Gaffer.

I was unsure whether to call the Pointer class Cursor or Pointer - cursor seems quite common but I associate that quite strongly with the text cursor. Happy to change the name if people think Cursor or anything else is better though.

Also if anyone with artistic abilities would like to improve the icons then please feel free. Oh, and I made the search icon less sinister :)