Add wxEXPAND spacers to frame wxBoxSizer on the top and bottom around
the drawing panel so that panel is correctly centered if maximum zoom is
set.

Set proportion to 1 on the spacers and 0 on the panel so that both
centering with maximum zoom and full expansion work correctly.

Refactor frame OnSize event: pass a dummy userdata pointer to
distinguish resizing the frame from resizing the panel, and call
Layout() when the frame is resized.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>