Getting the about icon to display in various scenarios on MacOSX
was a pain. Moving the icon to an include file solved the problem.
This commit also fixes the problem Dirk was having when converting
satellite.svg to a png in commit cf3c0266c2. I couldn't
quite get ImageMagick to preserve transparency and color when
converting subsurface-icon.svg, though, so I used Gimp instead.
Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
