Improve rendering of annotation squares.

[jpd236]

Use the same background rendering behavior as for regular white squares
instead of rendering as a black square, and don't render borders. This
matches the Crossword Nexus behavior.

[jpd236]

I could be convinced to leave the borders unchanged. IMO, they look nicer this way for Acrostics, and reasonable when in a grid surrounded by other (bordered) squares, but may look funkier in other situations.

[mrichards42]

This makes sense to me. It wouldn't be a bad idea to allow annotations to have borders, if we were ever to implement border styles, but I like this as a default. Annotation suggests to me that it's just printing some text in the grid, so you shouldn't expect it to look like a normal square with a border.