git ssb

Replace icon to avoid artifacts on dark background

The previous application icon looked great when displayed against
a white background (or similarly light colour), but it didn't look so
good against a dark background: jagged aliasing artifacts appeared.

This icon is a solid colour that fades to transparent, so will look OK
against dark and light backgrounds. It will, however, be difficult or
invisible against a background similar in colour to the icon itself (a
medium blue).