Fix two bugs in the display/gfx module:

1. The animation of the simple_menu used in the main menu had the issue
   that there is a black line visible at the top.  This is due the
   gfx_puts method ignoring lines, where the top pixel of the string is
   above the top of the screen.  As gfx_puts uses gfx_setpixel which in
   turn ignores pixels outside of the screen, remove the check in
   gfx_puts.
2. X and Y coordinates were cast to unsigned-ints before being given to
   the gfx-library which means calls like circ(0, -10, 30) would be draw
   at coordinates like [0,65526].  Fix this by changing the data-type of
   all coordinates to signed-integers.

Also remove the x and y ranges from the documentation of the individual
python functions and instead add a general documentation about the
screen and it's size/coordinate system.