Added a few display options, improvements in Menu class and other fixes.

- Added Display::setSize method to dynamically resize the display.
- Added a 'resizeable' argument in the display constructor, to control
whether the display will allow the user to resize it.
- Made the display instance available from the game instance, which now
is the recommended way to access the display object.
- Renamed Font::getFontHeight to Font::getHeight.
- Now the Menu class can be vertically scrolled. Its cursor can also be
wrapped around as well.
- Fixed Menu class entries' text incorrectly exceeding menu width. Also
fixed an incorrect internal behavior.
- Fixed Image::drawEllipse incorrect radii behavior on the SFML 1.6
adapter. (Finally!)