Window improvements and first kernel terminal tests.

[tmontes]

Window improvements:

• Cursor visibility configurable via constructor, defaults to hidden (addresses Windows should default to hiding the cursor. #1).
• Rendering produces zero bytes if cursor hidden and no output lines are produced (before it was still repositioning the cursor and more...)

Terminal tests:

• Initial rendering, re-rendering, and full re-rendering (contributing to Devise a way of testing the kernel. #9).